Add global config option for always_scrape_classic_histograms

Addresses https://github.com/prometheus/prometheus/issues/16371
This will help with migrating to native histograms with `convert_classic_histograms_to_nhcb` since users may still need to keep the classic histograms during a migration

Signed-off-by: chardch <otwordsne@gmail.com>

config/config.go

@@ -169,6 +169,7 @@ var (
 		// changes to DefaultNativeHistogramScrapeProtocols.
 		ScrapeProtocols:                DefaultScrapeProtocols,
 		ConvertClassicHistogramsToNHCB: false,
+		AlwaysScrapeClassicHistograms:  false,
 	}
 
 	DefaultRuntimeConfig = RuntimeConfig{
@@ -178,14 +179,13 @@ var (
 
 	// DefaultScrapeConfig is the default scrape configuration.
 	DefaultScrapeConfig = ScrapeConfig{
-		// ScrapeTimeout, ScrapeInterval and ScrapeProtocols default to the configured globals.
+		// ScrapeTimeout, ScrapeInterval, ScrapeProtocols, AlwaysScrapeClassicHistograms, and ConvertClassicHistogramsToNHCB default to the configured globals.
-		AlwaysScrapeClassicHistograms: false,
+		MetricsPath:       "/metrics",
-		MetricsPath:                   "/metrics",
+		Scheme:            "http",
-		Scheme:                        "http",
+		HonorLabels:       false,
-		HonorLabels:                   false,
+		HonorTimestamps:   true,
-		HonorTimestamps:               true,
+		HTTPClientConfig:  config.DefaultHTTPClientConfig,
-		HTTPClientConfig:              config.DefaultHTTPClientConfig,
+		EnableCompression: true,
-		EnableCompression:             true,
 	}
 
 	// DefaultAlertmanagerConfig is the default alertmanager configuration.
@@ -489,6 +489,8 @@ type GlobalConfig struct {
 	MetricNameEscapingScheme string `yaml:"metric_name_escaping_scheme,omitempty"`
 	// Whether to convert all scraped classic histograms into native histograms with custom buckets.
 	ConvertClassicHistogramsToNHCB bool `yaml:"convert_classic_histograms_to_nhcb,omitempty"`
+	// Whether to scrape a classic histogram, even if it is also exposed as a native histogram.
+	AlwaysScrapeClassicHistograms bool `yaml:"always_scrape_classic_histograms,omitempty"`
 }
 
 // ScrapeProtocol represents supported protocol for scraping metrics.
@@ -645,7 +647,8 @@ func (c *GlobalConfig) isZero() bool {
 		c.QueryLogFile == "" &&
 		c.ScrapeFailureLogFile == "" &&
 		c.ScrapeProtocols == nil &&
-		!c.ConvertClassicHistogramsToNHCB
+		!c.ConvertClassicHistogramsToNHCB &&
+		!c.AlwaysScrapeClassicHistograms
 }
 
 // RuntimeConfig configures the values for the process behavior.
@@ -690,7 +693,7 @@ type ScrapeConfig struct {
 	// OpenMetricsText1.0.0, PrometheusText1.0.0, PrometheusText0.0.4.
 	ScrapeFallbackProtocol ScrapeProtocol `yaml:"fallback_scrape_protocol,omitempty"`
 	// Whether to scrape a classic histogram, even if it is also exposed as a native histogram.
-	AlwaysScrapeClassicHistograms bool `yaml:"always_scrape_classic_histograms,omitempty"`
+	AlwaysScrapeClassicHistograms *bool `yaml:"always_scrape_classic_histograms,omitempty"`
 	// Whether to convert all scraped classic histograms into a native histogram with custom buckets.
 	ConvertClassicHistogramsToNHCB *bool `yaml:"convert_classic_histograms_to_nhcb,omitempty"`
 	// File to which scrape failures are logged.
@@ -904,6 +907,11 @@ func (c *ScrapeConfig) Validate(globalConfig GlobalConfig) error {
 		c.ConvertClassicHistogramsToNHCB = &global
 	}
 
+	if c.AlwaysScrapeClassicHistograms == nil {
+		global := globalConfig.AlwaysScrapeClassicHistograms
+		c.AlwaysScrapeClassicHistograms = &global
+	}
+
 	return nil
 }
 
@@ -931,6 +939,11 @@ func (c *ScrapeConfig) ConvertClassicHistogramsToNHCBEnabled() bool {
 	return c.ConvertClassicHistogramsToNHCB != nil && *c.ConvertClassicHistogramsToNHCB
 }
 
+// AlwaysScrapeClassicHistogramsEnabled returns whether to always scrape classic histograms.
+func (c *ScrapeConfig) AlwaysScrapeClassicHistogramsEnabled() bool {
+	return c.AlwaysScrapeClassicHistograms != nil && *c.AlwaysScrapeClassicHistograms
+}
+
 // StorageConfig configures runtime reloadable configuration options.
 type StorageConfig struct {
 	TSDBConfig      *TSDBConfig      `yaml:"tsdb,omitempty"`

config/config_test.go

@@ -102,6 +102,7 @@ var expectedConf = &Config{
 		LabelNameLengthLimit:           globLabelNameLengthLimit,
 		LabelValueLengthLimit:          globLabelValueLengthLimit,
 		ScrapeProtocols:                DefaultGlobalConfig.ScrapeProtocols,
+		AlwaysScrapeClassicHistograms:  false,
 		ConvertClassicHistogramsToNHCB: false,
 	},
 
@@ -223,6 +224,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           "testdata/fail_prom.log",
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath: DefaultScrapeConfig.MetricsPath,
@@ -339,6 +341,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			HTTPClientConfig: config.HTTPClientConfig{
@@ -440,6 +443,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -499,6 +503,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath: "/metrics",
@@ -536,6 +541,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -579,6 +585,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath: DefaultScrapeConfig.MetricsPath,
@@ -622,6 +629,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -655,6 +663,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -696,6 +705,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -734,6 +744,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -779,6 +790,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -814,6 +826,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -852,6 +865,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -883,6 +897,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -917,6 +932,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      "/federate",
@@ -951,6 +967,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -985,6 +1002,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1016,6 +1034,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1055,6 +1074,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1093,6 +1113,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1128,6 +1149,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1162,6 +1184,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1200,6 +1223,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1241,6 +1265,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1301,6 +1326,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1332,6 +1358,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			HTTPClientConfig: config.DefaultHTTPClientConfig,
@@ -1374,6 +1401,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			HTTPClientConfig: config.DefaultHTTPClientConfig,
@@ -1422,6 +1450,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1461,6 +1490,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			HTTPClientConfig: config.DefaultHTTPClientConfig,
@@ -1495,6 +1525,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1531,6 +1562,7 @@ var expectedConf = &Config{
 			ScrapeFailureLogFile:           globScrapeFailureLogFile,
 			MetricNameValidationScheme:     UTF8ValidationConfig,
 			MetricNameEscapingScheme:       model.AllowUTF8,
+			AlwaysScrapeClassicHistograms:  boolPtr(false),
 			ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -2353,13 +2385,23 @@ func TestEmptyGlobalBlock(t *testing.T) {
 	require.Equal(t, exp, *c)
 }
 
+// ScrapeConfigOptions contains options for creating a scrape config.
+type ScrapeConfigOptions struct {
+	JobName                       string
+	ScrapeInterval                model.Duration
+	ScrapeTimeout                 model.Duration
+	AlwaysScrapeClassicHistograms bool
+	ConvertClassicHistToNHCB      bool
+}
+
 func TestGetScrapeConfigs(t *testing.T) {
-	sc := func(jobName string, scrapeInterval, scrapeTimeout model.Duration, convertClassicHistToNHCB bool) *ScrapeConfig {
+	// Helper function to create a scrape config with the given options.
+	sc := func(opts ScrapeConfigOptions) *ScrapeConfig {
 		return &ScrapeConfig{
-			JobName:                    jobName,
+			JobName:                    opts.JobName,
 			HonorTimestamps:            true,
-			ScrapeInterval:             scrapeInterval,
+			ScrapeInterval:             opts.ScrapeInterval,
-			ScrapeTimeout:              scrapeTimeout,
+			ScrapeTimeout:              opts.ScrapeTimeout,
 			ScrapeProtocols:            DefaultGlobalConfig.ScrapeProtocols,
 			MetricNameValidationScheme: UTF8ValidationConfig,
 			MetricNameEscapingScheme:   model.AllowUTF8,
@@ -2380,7 +2422,8 @@ func TestGetScrapeConfigs(t *testing.T) {
 					},
 				},
 			},
-			ConvertClassicHistogramsToNHCB: boolPtr(convertClassicHistToNHCB),
+			AlwaysScrapeClassicHistograms:  boolPtr(opts.AlwaysScrapeClassicHistograms),
+			ConvertClassicHistogramsToNHCB: boolPtr(opts.ConvertClassicHistToNHCB),
 		}
 	}
 
@@ -2393,20 +2436,20 @@ func TestGetScrapeConfigs(t *testing.T) {
 		{
 			name:           "An included config file should be a valid global config.",
 			configFile:     "testdata/scrape_config_files.good.yml",
-			expectedResult: []*ScrapeConfig{sc("prometheus", model.Duration(60*time.Second), model.Duration(10*time.Second), false)},
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false})},
 		},
 		{
 			name:           "A global config that only include a scrape config file.",
 			configFile:     "testdata/scrape_config_files_only.good.yml",
-			expectedResult: []*ScrapeConfig{sc("prometheus", model.Duration(60*time.Second), model.Duration(10*time.Second), false)},
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false})},
 		},
 		{
 			name:       "A global config that combine scrape config files and scrape configs.",
 			configFile: "testdata/scrape_config_files_combined.good.yml",
 			expectedResult: []*ScrapeConfig{
-				sc("node", model.Duration(60*time.Second), model.Duration(10*time.Second), false),
+				sc(ScrapeConfigOptions{JobName: "node", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false}),
-				sc("prometheus", model.Duration(60*time.Second), model.Duration(10*time.Second), false),
+				sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false}),
-				sc("alertmanager", model.Duration(60*time.Second), model.Duration(10*time.Second), false),
+				sc(ScrapeConfigOptions{JobName: "alertmanager", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false}),
 			},
 		},
 		{
@@ -2422,6 +2465,7 @@ func TestGetScrapeConfigs(t *testing.T) {
 					ScrapeProtocols:                DefaultGlobalConfig.ScrapeProtocols,
 					MetricNameValidationScheme:     UTF8ValidationConfig,
 					MetricNameEscapingScheme:       model.AllowUTF8,
+					AlwaysScrapeClassicHistograms:  boolPtr(false),
 					ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 					MetricsPath: DefaultScrapeConfig.MetricsPath,
@@ -2458,6 +2502,7 @@ func TestGetScrapeConfigs(t *testing.T) {
 					ScrapeProtocols:                DefaultGlobalConfig.ScrapeProtocols,
 					MetricNameValidationScheme:     UTF8ValidationConfig,
 					MetricNameEscapingScheme:       model.AllowUTF8,
+					AlwaysScrapeClassicHistograms:  boolPtr(false),
 					ConvertClassicHistogramsToNHCB: boolPtr(false),
 
 					HTTPClientConfig: config.HTTPClientConfig{
@@ -2509,17 +2554,37 @@ func TestGetScrapeConfigs(t *testing.T) {
 		{
 			name:           "A global config that enables convert classic histograms to nhcb.",
 			configFile:     "testdata/global_convert_classic_hist_to_nhcb.good.yml",
-			expectedResult: []*ScrapeConfig{sc("prometheus", model.Duration(60*time.Second), model.Duration(10*time.Second), true)},
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: true})},
 		},
 		{
 			name:           "A global config that enables convert classic histograms to nhcb and scrape config that disables the conversion",
 			configFile:     "testdata/local_disable_convert_classic_hist_to_nhcb.good.yml",
-			expectedResult: []*ScrapeConfig{sc("prometheus", model.Duration(60*time.Second), model.Duration(10*time.Second), false)},
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false})},
 		},
 		{
 			name:           "A global config that disables convert classic histograms to nhcb and scrape config that enables the conversion",
 			configFile:     "testdata/local_convert_classic_hist_to_nhcb.good.yml",
-			expectedResult: []*ScrapeConfig{sc("prometheus", model.Duration(60*time.Second), model.Duration(10*time.Second), true)},
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: true})},
+		},
+		{
+			name:           "A global config that enables always scrape classic histograms",
+			configFile:     "testdata/global_enable_always_scrape_classic_hist.good.yml",
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: true, ConvertClassicHistToNHCB: false})},
+		},
+		{
+			name:           "A global config that disables always scrape classic histograms",
+			configFile:     "testdata/global_disable_always_scrape_classic_hist.good.yml",
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false})},
+		},
+		{
+			name:           "A global config that disables always scrape classic histograms and scrape config that enables it",
+			configFile:     "testdata/local_enable_always_scrape_classic_hist.good.yml",
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: true, ConvertClassicHistToNHCB: false})},
+		},
+		{
+			name:           "A global config that enables always scrape classic histograms and scrape config that disables it",
+			configFile:     "testdata/local_disable_always_scrape_classic_hist.good.yml",
+			expectedResult: []*ScrapeConfig{sc(ScrapeConfigOptions{JobName: "prometheus", ScrapeInterval: model.Duration(60 * time.Second), ScrapeTimeout: model.Duration(10 * time.Second), AlwaysScrapeClassicHistograms: false, ConvertClassicHistToNHCB: false})},
 		},
 	}
 

config/testdata/global_disable_always_scrape_classic_hist.good.yml

@@ -0,0 +1,6 @@
+global:
+  always_scrape_classic_histograms: false
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: ['localhost:8080']

config/testdata/global_enable_always_scrape_classic_hist.good.yml

@@ -0,0 +1,6 @@
+global:
+  always_scrape_classic_histograms: true
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: ['localhost:8080']

config/testdata/local_disable_always_scrape_classic_hist.good.yml

@@ -0,0 +1,7 @@
+global:
+  always_scrape_classic_histograms: true
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: ['localhost:8080']
+    always_scrape_classic_histograms: false

config/testdata/local_enable_always_scrape_classic_hist.good.yml

@@ -0,0 +1,7 @@
+global:
+  always_scrape_classic_histograms: false
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: ['localhost:8080']
+    always_scrape_classic_histograms: true

docs/configuration/configuration.md

@@ -144,6 +144,11 @@ global:
   # histograms with custom buckets.
   [ convert_classic_histograms_to_nhcb <bool> | default = false]
 
+  # Specifies whether to scrape a classic histogram, even if it is also exposed as a native
+  # histogram (has no effect without --enable-feature=native-histograms).
+  [ always_scrape_classic_histograms: <boolean> | default = false ]
+
+
 runtime:
   # Configure the Go garbage collector GOGC parameter
   # See: https://tip.golang.org/doc/gc-guide#GOGC
@@ -244,7 +249,8 @@ job_name: <job_name>
 
 # Whether to scrape a classic histogram, even if it is also exposed as a native
 # histogram (has no effect without --enable-feature=native-histograms).
-[ always_scrape_classic_histograms: <boolean> | default = false ]
+[ always_scrape_classic_histograms: <boolean> |
+default = <global.always_scrape_classic_hisotgrams> ]
 
 # The HTTP resource path on which to fetch metrics from targets.
 [ metrics_path: <path> | default = /metrics ]

scrape/scrape.go

@@ -366,7 +366,7 @@ func (sp *scrapePool) restartLoops(reuseCache bool) {
 		trackTimestampsStaleness = sp.config.TrackTimestampsStaleness
 		mrc                      = sp.config.MetricRelabelConfigs
 		fallbackScrapeProtocol   = sp.config.ScrapeFallbackProtocol.HeaderMediaType()
-		alwaysScrapeClassicHist  = sp.config.AlwaysScrapeClassicHistograms
+		alwaysScrapeClassicHist  = sp.config.AlwaysScrapeClassicHistogramsEnabled()
 		convertClassicHistToNHCB = sp.config.ConvertClassicHistogramsToNHCBEnabled()
 	)
 
@@ -522,7 +522,7 @@ func (sp *scrapePool) sync(targets []*Target) {
 		trackTimestampsStaleness = sp.config.TrackTimestampsStaleness
 		mrc                      = sp.config.MetricRelabelConfigs
 		fallbackScrapeProtocol   = sp.config.ScrapeFallbackProtocol.HeaderMediaType()
-		alwaysScrapeClassicHist  = sp.config.AlwaysScrapeClassicHistograms
+		alwaysScrapeClassicHist  = sp.config.AlwaysScrapeClassicHistogramsEnabled()
 		convertClassicHistToNHCB = sp.config.ConvertClassicHistogramsToNHCBEnabled()
 	)
 

scrape/scrape_test.go

@@ -4635,26 +4635,26 @@ metric: <
 	fals := false
 	for metricsTextName, metricsText := range metricsTexts {
 		for name, tc := range map[string]struct {
-			alwaysScrapeClassicHistograms bool
+			alwaysScrapeClassicHistograms *bool
 			convertClassicHistToNHCB      *bool
 		}{
 			"convert with scrape": {
-				alwaysScrapeClassicHistograms: true,
+				alwaysScrapeClassicHistograms: &tru,
 				convertClassicHistToNHCB:      &tru,
 			},
 			"convert without scrape": {
-				alwaysScrapeClassicHistograms: false,
+				alwaysScrapeClassicHistograms: &fals,
 				convertClassicHistToNHCB:      &tru,
 			},
 			"scrape without convert": {
-				alwaysScrapeClassicHistograms: true,
+				alwaysScrapeClassicHistograms: &tru,
 				convertClassicHistToNHCB:      &fals,
 			},
 			"scrape with nil convert": {
-				alwaysScrapeClassicHistograms: true,
+				alwaysScrapeClassicHistograms: &tru,
 			},
 			"neither scrape nor convert": {
-				alwaysScrapeClassicHistograms: false,
+				alwaysScrapeClassicHistograms: &fals,
 				convertClassicHistToNHCB:      &fals,
 			},
 		} {
@@ -4664,7 +4664,7 @@ metric: <
 				expectedNativeHistCount = 1
 				expectCustomBuckets = false
 				expectedClassicHistCount = 0
-				if metricsText.hasClassic && tc.alwaysScrapeClassicHistograms {
+				if metricsText.hasClassic && tc.alwaysScrapeClassicHistograms != nil && *tc.alwaysScrapeClassicHistograms {
 					expectedClassicHistCount = 1
 				}
 			} else if metricsText.hasClassic {
@@ -4672,11 +4672,11 @@ metric: <
 				case tc.convertClassicHistToNHCB == nil || !*tc.convertClassicHistToNHCB:
 					expectedClassicHistCount = 1
 					expectedNativeHistCount = 0
-				case tc.alwaysScrapeClassicHistograms && *tc.convertClassicHistToNHCB:
+				case tc.alwaysScrapeClassicHistograms != nil && *tc.alwaysScrapeClassicHistograms && *tc.convertClassicHistToNHCB:
 					expectedClassicHistCount = 1
 					expectedNativeHistCount = 1
 					expectCustomBuckets = true
-				case !tc.alwaysScrapeClassicHistograms && *tc.convertClassicHistToNHCB:
+				case (tc.alwaysScrapeClassicHistograms == nil || !*tc.alwaysScrapeClassicHistograms) && *tc.convertClassicHistToNHCB:
 					expectedClassicHistCount = 0
 					expectedNativeHistCount = 1
 					expectCustomBuckets = true