tsdb: Fix appended sample count metrics when converting float staleness markers to histograms (#17241)

tsdb: Fix appended sample count metrics when converting histogram staleness markers

Signed-off-by: Patryk Prus <p@trykpr.us>
Signed-off-by: Björn Rabenstein <github@rabenste.in>
Co-authored-by: Björn Rabenstein <github@rabenste.in>

tsdb/head_append.go

@@ -1331,6 +1331,9 @@ func (a *headAppender) commitFloats(b *appendBatch, acc *appenderCommitContext)
 					H:   &histogram.Histogram{Sum: s.V},
 				})
 				b.histogramSeries = append(b.histogramSeries, series)
+				// This sample was counted as a float but is now a histogram.
+				acc.floatsAppended--
+				acc.histogramsAppended++
 				series.Unlock()
 				continue
 			case series.lastFloatHistogramValue != nil:
@@ -1340,6 +1343,9 @@ func (a *headAppender) commitFloats(b *appendBatch, acc *appenderCommitContext)
 					FH:  &histogram.FloatHistogram{Sum: s.V},
 				})
 				b.floatHistogramSeries = append(b.floatHistogramSeries, series)
+				// This sample was counted as a float but is now a float histogram.
+				acc.floatsAppended--
+				acc.histogramsAppended++
 				series.Unlock()
 				continue
 			}

tsdb/head_test.go

@@ -34,6 +34,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"github.com/prometheus/client_golang/prometheus"
 	prom_testutil "github.com/prometheus/client_golang/prometheus/testutil"
+	dto "github.com/prometheus/client_model/go"
 	"github.com/prometheus/common/promslog"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/atomic"
@@ -7256,3 +7257,92 @@ func TestHead_NumStaleSeries(t *testing.T) {
 	appendFloatHistogram(series8, 400, staleFH)
 	verifySeriesCounts(4, 5)
 }
+
+// TestHistogramStalenessConversionMetrics verifies that staleness marker conversion correctly
+// increments the right appender metrics for both histogram and float histogram scenarios.
+func TestHistogramStalenessConversionMetrics(t *testing.T) {
+	testCases := []struct {
+		name           string
+		setupHistogram func(app storage.Appender, lbls labels.Labels) error
+	}{
+		{
+			name: "float_staleness_to_histogram",
+			setupHistogram: func(app storage.Appender, lbls labels.Labels) error {
+				_, err := app.AppendHistogram(0, lbls, 1000, tsdbutil.GenerateTestHistograms(1)[0], nil)
+				return err
+			},
+		},
+		{
+			name: "float_staleness_to_float_histogram",
+			setupHistogram: func(app storage.Appender, lbls labels.Labels) error {
+				_, err := app.AppendHistogram(0, lbls, 1000, nil, tsdbutil.GenerateTestFloatHistograms(1)[0])
+				return err
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			head, _ := newTestHead(t, 1000, compression.None, false)
+			defer func() {
+				require.NoError(t, head.Close())
+			}()
+
+			lbls := labels.FromStrings("name", tc.name)
+
+			// Helper to get counter values
+			getSampleCounter := func(sampleType string) float64 {
+				metric := &dto.Metric{}
+				err := head.metrics.samplesAppended.WithLabelValues(sampleType).Write(metric)
+				require.NoError(t, err)
+				return metric.GetCounter().GetValue()
+			}
+
+			// Step 1: Establish a series with histogram data
+			app := head.Appender(context.Background())
+			err := tc.setupHistogram(app, lbls)
+			require.NoError(t, err)
+			require.NoError(t, app.Commit())
+
+			// Step 2: Add a float staleness marker
+			app = head.Appender(context.Background())
+			_, err = app.Append(0, lbls, 2000, math.Float64frombits(value.StaleNaN))
+			require.NoError(t, err)
+			require.NoError(t, app.Commit())
+
+			// Count what was actually stored by querying the series
+			q, err := NewBlockQuerier(head, 0, 3000)
+			require.NoError(t, err)
+			defer q.Close()
+
+			ss := q.Select(context.Background(), false, nil, labels.MustNewMatcher(labels.MatchEqual, "name", tc.name))
+			require.True(t, ss.Next())
+			series := ss.At()
+
+			it := series.Iterator(nil)
+
+			actualFloatSamples := 0
+			actualHistogramSamples := 0
+
+			for valType := it.Next(); valType != chunkenc.ValNone; valType = it.Next() {
+				switch valType {
+				case chunkenc.ValFloat:
+					actualFloatSamples++
+				case chunkenc.ValHistogram, chunkenc.ValFloatHistogram:
+					actualHistogramSamples++
+				}
+			}
+			require.NoError(t, it.Err())
+
+			// Verify what was actually stored - should be 0 floats, 2 histograms (original + converted staleness marker)
+			require.Equal(t, 0, actualFloatSamples, "Should have 0 float samples stored")
+			require.Equal(t, 2, actualHistogramSamples, "Should have 2 histogram samples: original + converted staleness marker")
+
+			// The metrics should match what was actually stored
+			require.Equal(t, float64(actualFloatSamples), getSampleCounter(sampleMetricTypeFloat),
+				"Float counter should match actual float samples stored")
+			require.Equal(t, float64(actualHistogramSamples), getSampleCounter(sampleMetricTypeHistogram),
+				"Histogram counter should match actual histogram samples stored")
+		})
+	}
+}