[receiver/prometheus] Don't drop Histograms without buckets (#23448)

**Description:**
Prometheus receiver currently drops Histograms without any buckets.
These are, however, explicitly allowed by the Otel spec, and can be
quite useful. This change allows ingesting them. When we do so, we add
an additional bucket at +Inf equal to the `count` attribute of the
Histogram.

**Link to tracking Issue:** #22070 

**Testing:**
Modified existing tests.

.chloggen/fix_prometheusreceiver_empty-histogram.yaml

@@ -0,0 +1,20 @@
+# Use this changelog template to create an entry for release notes.
+# If your change doesn't affect end users, such as a test fix or a tooling change,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: prometheusreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Don't drop histograms without buckets
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [22070]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

receiver/prometheusreceiver/internal/metricfamily.go

@@ -87,27 +87,28 @@ func (mg *metricGroup) sortPoints() {
 }
 
 func (mg *metricGroup) toDistributionPoint(dest pmetric.HistogramDataPointSlice) {
-	if !mg.hasCount || len(mg.complexValue) == 0 {
+	if !mg.hasCount {
 		return
 	}
 
 	mg.sortPoints()
 
+	bucketCount := len(mg.complexValue) + 1
+	// if the final bucket is +Inf, we ignore it
+	if bucketCount > 1 && mg.complexValue[bucketCount-2].boundary == math.Inf(1) {
+		bucketCount--
+	}
+
 	// for OTLP the bounds won't include +inf
-	bounds := make([]float64, len(mg.complexValue)-1)
-	bucketCounts := make([]uint64, len(mg.complexValue))
+	bounds := make([]float64, bucketCount-1)
+	bucketCounts := make([]uint64, bucketCount)
+	var adjustedCount float64
 
 	pointIsStale := value.IsStaleNaN(mg.sum) || value.IsStaleNaN(mg.count)
+	for i := 0; i < bucketCount-1; i++ {
+		bounds[i] = mg.complexValue[i].boundary
+		adjustedCount = mg.complexValue[i].value
 
-	for i := 0; i < len(mg.complexValue); i++ {
-		if i != len(mg.complexValue)-1 {
-			// not need to add +inf as OTLP assumes it
-			bounds[i] = mg.complexValue[i].boundary
-		} else if mg.complexValue[i].boundary != math.Inf(1) {
-			// This histogram is missing the +Inf bucket, and isn't a complete prometheus histogram.
-			return
-		}
-		adjustedCount := mg.complexValue[i].value
 		// Buckets still need to be sent to know to set them as stale,
 		// but a staleness NaN converted to uint64 would be an extremely large number.
 		// Setting to 0 instead.
@@ -119,6 +120,15 @@ func (mg *metricGroup) toDistributionPoint(dest pmetric.HistogramDataPointSlice)
 		bucketCounts[i] = uint64(adjustedCount)
 	}
 
+	// Add the final bucket based on the total count
+	adjustedCount = mg.count
+	if pointIsStale {
+		adjustedCount = 0
+	} else if bucketCount > 1 {
+		adjustedCount -= mg.complexValue[bucketCount-2].value
+	}
+	bucketCounts[bucketCount-1] = uint64(adjustedCount)
+
 	point := dest.AppendEmpty()
 
 	if pointIsStale {

receiver/prometheusreceiver/internal/metricfamily_test.go

@@ -218,6 +218,28 @@ func TestMetricGroupData_toDistributionUnitTest(t *testing.T) {
 			},
 			wantErr: true,
 		},
+		{
+			name:                "histogram without buckets",
+			metricName:          "histogram",
+			intervalStartTimeMs: 11,
+			labels:              labels.FromMap(map[string]string{"a": "A", "b": "B"}),
+			scrapes: []*scrape{
+				{at: 11, value: 66, metric: "histogram_count"},
+				{at: 11, value: 1004.78, metric: "histogram_sum"},
+			},
+			want: func() pmetric.HistogramDataPoint {
+				point := pmetric.NewHistogramDataPoint()
+				point.SetCount(66)
+				point.SetSum(1004.78)
+				point.SetTimestamp(pcommon.Timestamp(11 * time.Millisecond))      // the time in milliseconds -> nanoseconds.
+				point.SetStartTimestamp(pcommon.Timestamp(11 * time.Millisecond)) // the time in milliseconds -> nanoseconds.
+				point.BucketCounts().FromRaw([]uint64{66})
+				attributes := point.Attributes()
+				attributes.PutStr("a", "A")
+				attributes.PutStr("b", "B")
+				return point
+			},
+		},
 	}
 
 	for _, tt := range tests {

receiver/prometheusreceiver/internal/transaction_test.go

@@ -1218,6 +1218,20 @@ func TestMetricBuilderHistogram(t *testing.T) {
 			},
 			wants: func() []pmetric.Metrics {
 				md0 := pmetric.NewMetrics()
+				mL0 := md0.ResourceMetrics().AppendEmpty().ScopeMetrics().AppendEmpty().Metrics()
+				m0 := mL0.AppendEmpty()
+				m0.SetName("hist_test")
+				hist0 := m0.SetEmptyHistogram()
+				hist0.SetAggregationTemporality(pmetric.AggregationTemporalityCumulative)
+				pt0 := hist0.DataPoints().AppendEmpty()
+				pt0.SetCount(3)
+				pt0.SetSum(100)
+				pt0.BucketCounts().FromRaw([]uint64{3, 0})
+				pt0.ExplicitBounds().FromRaw([]float64{20})
+				pt0.SetTimestamp(tsNanos)
+				pt0.SetStartTimestamp(startTimestamp)
+				pt0.Attributes().PutStr("foo", "bar")
+
 				return []pmetric.Metrics{md0}
 			},
 		},
@@ -1256,13 +1270,27 @@ func TestMetricBuilderHistogram(t *testing.T) {
 			inputs: []*testScrapedPage{
 				{
 					pts: []*testDataPoint{
-						createDataPoint("hist_test_sum", 99, nil),
-						createDataPoint("hist_test_count", 10, nil),
+						createDataPoint("hist_test_sum", 99, nil, "foo", "bar"),
+						createDataPoint("hist_test_count", 10, nil, "foo", "bar"),
 					},
 				},
 			},
 			wants: func() []pmetric.Metrics {
-				return []pmetric.Metrics{pmetric.NewMetrics()}
+				md0 := pmetric.NewMetrics()
+				mL0 := md0.ResourceMetrics().AppendEmpty().ScopeMetrics().AppendEmpty().Metrics()
+				m0 := mL0.AppendEmpty()
+				m0.SetName("hist_test")
+				hist0 := m0.SetEmptyHistogram()
+				hist0.SetAggregationTemporality(pmetric.AggregationTemporalityCumulative)
+				pt0 := hist0.DataPoints().AppendEmpty()
+				pt0.SetCount(10)
+				pt0.SetSum(99)
+				pt0.BucketCounts().FromRaw([]uint64{10})
+				pt0.SetTimestamp(tsNanos)
+				pt0.SetStartTimestamp(startTimestamp)
+				pt0.Attributes().PutStr("foo", "bar")
+
+				return []pmetric.Metrics{md0}
 			},
 		},
 		{

receiver/prometheusreceiver/metrics_receiver_test.go

@@ -1087,8 +1087,8 @@ foo_total 1
 func verifyTarget3(t *testing.T, td *testData, resourceMetrics []pmetric.ResourceMetrics) {
 	verifyNumValidScrapeResults(t, td, resourceMetrics)
 	m1 := resourceMetrics[0]
-	// m1 has 3 metrics + 5 internal scraper metrics
-	assert.Equal(t, 8, metricsCount(m1))
+	// m1 has 4 metrics + 5 internal scraper metrics
+	assert.Equal(t, 9, metricsCount(m1))
 
 	wantAttributes := td.attributes
 
@@ -1116,7 +1116,17 @@ func verifyTarget3(t *testing.T, td *testData, resourceMetrics []pmetric.Resourc
 					},
 				},
 			}),
-		assertMetricAbsent("corrupted_hist"),
+		assertMetricPresent("corrupted_hist",
+			compareMetricType(pmetric.MetricTypeHistogram),
+			[]dataPointExpectation{
+				{
+					histogramPointComparator: []histogramPointComparator{
+						compareHistogramStartTimestamp(ts1),
+						compareHistogramTimestamp(ts1),
+						compareHistogram(10, 100, []uint64{10}),
+					},
+				},
+			}),
 		assertMetricPresent("rpc_duration_seconds",
 			compareMetricType(pmetric.MetricTypeSummary),
 			[]dataPointExpectation{
@@ -1141,8 +1151,8 @@ func verifyTarget3(t *testing.T, td *testData, resourceMetrics []pmetric.Resourc
 	doCompare(t, "scrape1", wantAttributes, m1, e1)
 
 	m2 := resourceMetrics[1]
-	// m2 has 3 metrics + 5 internal scraper metrics
-	assert.Equal(t, 8, metricsCount(m2))
+	// m2 has 4 metrics + 5 internal scraper metrics
+	assert.Equal(t, 9, metricsCount(m2))
 
 	metricsScrape2 := m2.ScopeMetrics().At(0).Metrics()
 	ts2 := getTS(metricsScrape2)
@@ -1168,7 +1178,17 @@ func verifyTarget3(t *testing.T, td *testData, resourceMetrics []pmetric.Resourc
 					},
 				},
 			}),
-		assertMetricAbsent("corrupted_hist"),
+		assertMetricPresent("corrupted_hist",
+			compareMetricType(pmetric.MetricTypeHistogram),
+			[]dataPointExpectation{
+				{
+					histogramPointComparator: []histogramPointComparator{
+						compareHistogramStartTimestamp(ts1),
+						compareHistogramTimestamp(ts2),
+						compareHistogram(15, 101, []uint64{15}),
+					},
+				},
+			}),
 		assertMetricPresent("rpc_duration_seconds",
 			compareMetricType(pmetric.MetricTypeSummary),
 			[]dataPointExpectation{