util: add +Inf bucket in MetricFamiliesToWriteRequest when not present (#15864)

* Add +Inf bucket in MetricFamiliesToWriteRequest if not present

Signed-off-by: Clark McCauley <clarkmccauley@gmail.com>

util/fmtutil/format.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"io"
 	"maps"
+	"math"
 	"sort"
 	"time"
 
@@ -140,11 +141,23 @@ func makeTimeseries(wr *prompb.WriteRequest, labels map[string]string, m *dto.Me
 		// Add Histogram bucket timeseries
 		bucketLabels := make(map[string]string, len(labels)+1)
 		maps.Copy(bucketLabels, labels)
+		var hasInf bool
 		for _, b := range m.GetHistogram().Bucket {
+			if b.GetUpperBound() == math.Inf(1) {
+				hasInf = true
+			}
 			bucketLabels[model.MetricNameLabel] = metricName + bucketStr
 			bucketLabels[model.BucketLabel] = fmt.Sprint(b.GetUpperBound())
 			toTimeseries(wr, bucketLabels, timestamp, float64(b.GetCumulativeCount()))
 		}
+
+		// Add +Inf bucket if not present
+		if !hasInf {
+			bucketLabels[model.MetricNameLabel] = metricName + bucketStr
+			bucketLabels[model.BucketLabel] = "+Inf"
+			toTimeseries(wr, bucketLabels, timestamp, float64(m.GetHistogram().GetSampleCount()))
+		}
+
 		// Overwrite label model.MetricNameLabel for count and sum metrics
 		// Add Histogram sum timeseries
 		labels[model.MetricNameLabel] = metricName + sumStr

util/fmtutil/format_test.go

@@ -15,8 +15,11 @@ package fmtutil
 
 import (
 	"bytes"
+	"math"
 	"testing"
+	"time"
 
+	dto "github.com/prometheus/client_model/go"
 	"github.com/stretchr/testify/require"
 
 	"github.com/prometheus/prometheus/prompb"
@@ -231,3 +234,50 @@ func TestMetricTextToWriteRequestErrorParsingMetricType(t *testing.T) {
 	_, err := MetricTextToWriteRequest(input, labels)
 	require.Equal(t, "text format parsing error in line 3: unknown metric type \"info\"", err.Error())
 }
+
+func TestMakeTimeseries_HistogramInfBucket(t *testing.T) {
+	tests := map[string]*dto.Histogram{
+		"Histogram missing +Inf bucket": {
+			Bucket: []*dto.Bucket{
+				{CumulativeCount: p[uint64](5), UpperBound: p(1.0)},
+				{CumulativeCount: p[uint64](10), UpperBound: p(5.0)},
+			},
+			SampleCount: p[uint64](15),
+		},
+		"Histogram already has +Inf bucket": {
+			Bucket: []*dto.Bucket{
+				{CumulativeCount: p[uint64](5), UpperBound: p(1.0)},
+				{CumulativeCount: p[uint64](10), UpperBound: p(5.0)},
+				{CumulativeCount: p[uint64](15), UpperBound: p(math.Inf(1))},
+			},
+			SampleCount: p[uint64](15),
+		},
+	}
+
+	for name, histogram := range tests {
+		t.Run(name, func(t *testing.T) {
+			wr := &prompb.WriteRequest{}
+			labels := map[string]string{"__name__": "test_histogram"}
+			metric := &dto.Metric{
+				Histogram:   histogram,
+				TimestampMs: p(time.Now().UnixMilli()),
+			}
+
+			require.NoError(t, makeTimeseries(wr, labels, metric))
+
+			var hasInf bool
+			for _, ts := range wr.Timeseries {
+				for _, lbl := range ts.Labels {
+					if lbl.Name == "le" && lbl.Value == "+Inf" {
+						hasInf = true
+					}
+				}
+			}
+			require.Truef(t, hasInf, "expected +Inf bucket in histogram")
+		})
+	}
+}
+
+func p[T any](v T) *T {
+	return &v
+}