fix metrics type metadata when some quantiles are cleaned due to summary window

metrics.go

@@ -26,7 +26,11 @@ import (
 type namedMetric struct {
 	name   string
 	metric metric
-	isAux  bool
+
+	// isAux indicates whether it is an auxiliary metric.
+	// Currently only set for quantileValue, as it is part of the Summary.
+	// This field affects sorting when quantileValue and Summary are compared.
+	isAux bool
 }
 
 type metric interface {

set.go

@@ -50,12 +50,10 @@ func (s *Set) WritePrometheus(w io.Writer) {
 			return fName1 < fName2
 		}
 
-		// Only summary and quantile(s) have different metric types.
-		// Sorting by metric type will stabilize the order for summary and quantile(s).
-		mType1 := s.a[i].metric.metricType()
-		mType2 := s.a[j].metric.metricType()
-		if mType1 != mType2 {
-			return mType1 < mType2
+		// if both has same family, check the auxiliary first.
+		// only summary quantile(s) is registered as auxiliary, and quantile(s) in summary should go first.
+		if s.a[i].isAux != s.a[j].isAux {
+			return s.a[i].isAux
 		}
 
 		// lastly by metric names, which is for quantiles and histogram buckets.

set_test.go

@@ -1,6 +1,7 @@
 package metrics
 
 import (
+	"bytes"
 	"fmt"
 	"sync"
 	"testing"
@@ -171,3 +172,27 @@ func TestRegisterUnregister(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+func TestMetadataForAfterSummaryWindow(t *testing.T) {
+	ExposeMetadata(true)
+	defer ExposeMetadata(false)
+
+	s := NewSet()
+	var testSummaryQuantilesNoop = []float64{0.5, 0.9, 0.97, 0.99, 1}
+	testWindow := 50 * time.Millisecond
+	s.NewSummaryExt(`test_summary_expire_quick`, testWindow, testSummaryQuantilesNoop).Update(1)
+	time.Sleep(4 * testWindow)
+
+	var bb bytes.Buffer
+	s.WritePrometheus(&bb)
+
+	expect := `# HELP test_summary_expire_quick
+# TYPE test_summary_expire_quick summary
+test_summary_expire_quick_sum 1
+test_summary_expire_quick_count 1
+`
+
+	if bb.String() != expect {
+		t.Fatalf("unexpected summary metric names:\n%s", bb.String())
+	}
+}

summary.go

@@ -120,13 +120,7 @@ func (sm *Summary) marshalTo(prefix string, w io.Writer) {
 }
 
 func (sm *Summary) metricType() string {
-	// this metric type should not be printed, because summary (sum and count)
-	// of the same metric family will be printed after quantile(s).
-	// If metadata is needed, the metadata from quantile(s) should be used.
-	// quantile will be printed first, so its metrics type won't be printed as metadata.
-	// Printing quantiles before sum and count aligns this code with Prometheus behavior.
-	// See: https://github.com/VictoriaMetrics/metrics/pull/99
-	return "unsupported"
+	return "summary"
 }
 
 func splitMetricName(name string) (string, string) {