test(cmd/prometheus): add TestFeatureFlagsDocumented and adjust --enable-feature description (#18487)

* test(cmd/prometheus): add TestFeatureFlagsDocumented and fix help text

Add TestFeatureFlagsDocumented to ensure the --enable-feature help text
in and docs/feature_flags.md list the same set of flags.

The help text was out of sync with the documentation:
- Flags present in docs but missing from help text: `auto-reload-config`,
  `metadata-wal-records`, `otlp-native-delta-ingestion`,
  `promql-delayed-name-removal`, `type-and-unit-labels`. Added them.
- Flags present in help text but missing from docs: `auto-gomaxprocs`,
  `expand-external-labels`. Removed them.

The help text is now sorted for better readability and kept in sync
with the documentation.

Also, the parsing of an empty `--enable-feature` was changed to
print `msg="Unknown option for --enable-feature" option=""` instead of nothing.

Signed-off-by: Ayoub Mrini <ayoubmrini424@gmail.com>

* main.go remove default for --enable-feature to avoid unwanted

Signed-off-by: Ayoub Mrini <ayoubmrini424@gmail.com>

---------

Signed-off-by: Ayoub Mrini <ayoubmrini424@gmail.com>

cmd/prometheus/main.go

@@ -305,8 +305,6 @@ func (c *flagConfig) setFeatureListOptions(logger *slog.Logger) error {
 			case "promql-binop-fill-modifiers":
 				c.parserOpts.EnableBinopFillModifiers = true
 				logger.Info("Experimental PromQL binary operator fill modifiers enabled.")
-			case "":
-				continue
 			case "old-ui":
 				c.web.UseOldUI = true
 				logger.Info("Serving previous version of the Prometheus web UI.")
@@ -608,8 +606,8 @@ func main() {
 	a.Flag("scrape.discovery-reload-interval", "Interval used by scrape manager to throttle target groups updates.").
 		Hidden().Default("5s").SetValue(&cfg.scrape.DiscoveryReloadInterval)
 
-	a.Flag("enable-feature", "Comma separated feature names to enable. Valid options: exemplar-storage, expand-external-labels, memory-snapshot-on-shutdown, promql-per-step-stats, promql-experimental-functions, extra-scrape-metrics, auto-gomaxprocs, created-timestamp-zero-ingestion, st-storage, concurrent-rule-eval, delayed-compaction, old-ui, otlp-deltatocumulative, promql-duration-expr, use-uncached-io, promql-extended-range-selectors, promql-binop-fill-modifiers, xor2-encoding. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details.").
-		Default("").StringsVar(&cfg.featureList)
+	a.Flag("enable-feature", "Comma separated feature names to enable. Valid options: auto-reload-config, concurrent-rule-eval, created-timestamp-zero-ingestion, delayed-compaction, exemplar-storage, extra-scrape-metrics, memory-snapshot-on-shutdown, metadata-wal-records, old-ui, otlp-deltatocumulative, otlp-native-delta-ingestion, promql-binop-fill-modifiers, promql-delayed-name-removal, promql-duration-expr, promql-experimental-functions, promql-extended-range-selectors, promql-per-step-stats, st-storage, type-and-unit-labels, use-uncached-io, xor2-encoding. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details.").
+		StringsVar(&cfg.featureList)
 
 	a.Flag("agent", "Run Prometheus in 'Agent mode'.").BoolVar(&agentMode)
 

cmd/prometheus/main_test.go

@@ -26,6 +26,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -34,6 +35,7 @@ import (
 	"time"
 
 	"github.com/alecthomas/kingpin/v2"
+	"github.com/grafana/regexp"
 	remoteapi "github.com/prometheus/client_golang/exp/api/remote"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/expfmt"
@@ -1062,3 +1064,30 @@ remote_write:
 		// 3*shardUpdateDuration to allow for the resharding logic to run.
 	}, 30*time.Second, time.Second)
 }
+
+// TestFeatureFlagsDocumented ensures the --enable-feature help text in main.go
+// and the documented flags in docs/feature_flags.md list the same set of flags.
+func TestFeatureFlagsDocumented(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.SkipNow()
+	}
+	h, err := os.ReadFile(filepath.Join("..", "..", "cmd", "prometheus", "main.go"))
+	require.NoError(t, err)
+	m := regexp.MustCompile(`a\.Flag\("enable-feature", "Comma separated feature names to enable. Valid options: (.+?)\.`).FindSubmatch(h)
+	require.NotNil(t, m)
+	var helpFlags []string
+	for f := range strings.SplitSeq(string(m[1]), ",") {
+		helpFlags = append(helpFlags, strings.TrimSpace(f))
+	}
+	require.NotEmpty(t, helpFlags)
+
+	d, err := os.ReadFile(filepath.Join("..", "..", "docs", "feature_flags.md"))
+	require.NoError(t, err)
+	var docFlags []string
+	for _, dm := range regexp.MustCompile("(?m)^`--enable-feature=(.+)`$").FindAllSubmatch(d, -1) {
+		docFlags = append(docFlags, string(dm[1]))
+	}
+	require.NotEmpty(t, docFlags)
+	require.True(t, slices.IsSorted(helpFlags))
+	require.ElementsMatch(t, helpFlags, docFlags)
+}

docs/command-line/prometheus.md

@@ -59,7 +59,7 @@ The Prometheus monitoring server
 | <code class="text-nowrap">--query.timeout</code> | Maximum time a query may take before being aborted. Use with server mode only. | `2m` |
 | <code class="text-nowrap">--query.max-concurrency</code> | Maximum number of queries executed concurrently. Use with server mode only. | `20` |
 | <code class="text-nowrap">--query.max-samples</code> | Maximum number of samples a single query can load into memory. Note that queries will fail if they try to load more samples than this into memory, so this also limits the number of samples a query can return. Use with server mode only. | `50000000` |
-| <code class="text-nowrap">--enable-feature</code> <code class="text-nowrap">...<code class="text-nowrap"> | Comma separated feature names to enable. Valid options: exemplar-storage, expand-external-labels, memory-snapshot-on-shutdown, promql-per-step-stats, promql-experimental-functions, extra-scrape-metrics, auto-gomaxprocs, created-timestamp-zero-ingestion, st-storage, concurrent-rule-eval, delayed-compaction, old-ui, otlp-deltatocumulative, promql-duration-expr, use-uncached-io, promql-extended-range-selectors, promql-binop-fill-modifiers, xor2-encoding. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details. |  |
+| <code class="text-nowrap">--enable-feature</code> <code class="text-nowrap">...<code class="text-nowrap"> | Comma separated feature names to enable. Valid options: auto-reload-config, concurrent-rule-eval, created-timestamp-zero-ingestion, delayed-compaction, exemplar-storage, extra-scrape-metrics, memory-snapshot-on-shutdown, metadata-wal-records, old-ui, otlp-deltatocumulative, otlp-native-delta-ingestion, promql-binop-fill-modifiers, promql-delayed-name-removal, promql-duration-expr, promql-experimental-functions, promql-extended-range-selectors, promql-per-step-stats, st-storage, type-and-unit-labels, use-uncached-io, xor2-encoding. See https://prometheus.io/docs/prometheus/latest/feature_flags/ for more details. |  |
 | <code class="text-nowrap">--agent</code> | Run Prometheus in 'Agent mode'. |  |
 | <code class="text-nowrap">--log.level</code> | Only log messages with the given severity or above. One of: [debug, info, warn, error] | `info` |
 | <code class="text-nowrap">--log.format</code> | Output format of log messages. One of: [logfmt, json] | `logfmt` |