Allow out of order samples to be written to the WAL. (#5487)

* Allow out of order samples to be written to the WAL.

* changelog

CHANGELOG.md

@@ -44,6 +44,8 @@ Main (unreleased)
 
 - Allow converting labels to structured metadata with Loki's structured_metadata stage. (@gonzalesraul)
 
+- Allow Out of Order writing to the WAL for metrics. (@mattdurham)
+
 ### Other changes
 
 - Use Go 1.21.3 for builds. (@tpaschalis)

pkg/metrics/wal/wal.go

@@ -699,11 +699,6 @@ func (a *appender) Append(ref storage.SeriesRef, l labels.Labels, t int64, v flo
 	series.Lock()
 	defer series.Unlock()
 
-	if t < series.lastTs {
-		a.w.metrics.totalOutOfOrderSamples.Inc()
-		return 0, storage.ErrOutOfOrderSample
-	}
-
 	// NOTE(rfratto): always modify pendingSamples and sampleSeries together.
 	a.pendingSamples = append(a.pendingSamples, record.RefSample{
 		Ref: series.ref,
@@ -824,11 +819,6 @@ func (a *appender) AppendHistogram(ref storage.SeriesRef, l labels.Labels, t int
 	series.Lock()
 	defer series.Unlock()
 
-	if t < series.lastTs {
-		a.w.metrics.totalOutOfOrderSamples.Inc()
-		return 0, storage.ErrOutOfOrderSample
-	}
-
 	switch {
 	case h != nil:
 		// NOTE(rfratto): always modify pendingHistograms and histogramSeries

pkg/metrics/wal/wal_test.go

@@ -434,6 +434,35 @@ func TestGlobalReferenceID_Normal(t *testing.T) {
 	require.True(t, ref3 == 2)
 }
 
+func TestDBAllowOOOSamples(t *testing.T) {
+	walDir := t.TempDir()
+
+	s, err := NewStorage(log.NewNopLogger(), nil, walDir)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, s.Close())
+	}()
+
+	app := s.Appender(context.Background())
+
+	// Write some samples
+	payload := buildSeries([]string{"foo", "bar", "baz"})
+	for _, metric := range payload {
+		metric.Write(t, app)
+	}
+
+	require.NoError(t, app.Commit())
+	for _, metric := range payload {
+		for _, sa := range metric.samples {
+			// We want to set the timestamp to before. This should no longer trigger an out of order.
+			sa.ts = sa.ts - 10_000
+		}
+	}
+	for _, metric := range payload {
+		metric.Write(t, app)
+	}
+}
+
 func BenchmarkAppendExemplar(b *testing.B) {
 	walDir := b.TempDir()