[Exporter.Prometheus] Fix OpenMetrics histograms

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md

@@ -38,14 +38,18 @@ Notes](../../RELEASENOTES.md).
   ([#7219](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7219))
 
 * Fix Prometheus/OpenMetrics serialization to emit metric and label names
-  containing `:` and `_` instead of dropping them and prefixing leading digits.
+  containing `_` instead of dropping them and prefixing leading digits.
   Invalid characters are replaced with `_` instead of being dropped.
   ([#7209](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7209))
 
 * Add `escaping=underscores` to the `Accept` header handling for content
   negotiation so OpenMetrics are handled correctly.
   ([#7209](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7209))
 
+* Omit histogram `_sum` and `_count` in OpenMetrics when negative bucket
+  thresholds are present.
+  ([#7221](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7221))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md

@@ -55,14 +55,18 @@ Notes](../../RELEASENOTES.md).
   ([#7167](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7167))
 
 * Fix Prometheus/OpenMetrics serialization to emit metric and label names
-  containing `:` and `_` instead of dropping them and prefixing leading digits.
+  containing and `_` instead of dropping them and prefixing leading digits.
   Invalid characters are replaced with `_` instead of being dropped.
   ([#7209](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7209))
 
 * Add `escaping=underscores` to the `Accept` header handling for content
   negotiation so OpenMetrics are handled correctly.
   ([#7209](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7209))
 
+* Omit histogram `_sum` and `_count` in OpenMetrics when negative bucket
+  thresholds are present.
+  ([#7221](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7221))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs

@@ -89,11 +89,17 @@ public static int WriteMetric(
             foreach (ref readonly var metricPoint in metric.GetMetricPoints())
             {
                 var tags = metricPoint.Tags;
+                var hasNegativeBucketBounds = false;
                 var previousBound = double.NegativeInfinity;
 
                 long totalCount = 0;
                 foreach (var histogramMeasurement in metricPoint.GetHistogramBuckets())
                 {
+                    if (openMetricsRequested && histogramMeasurement.ExplicitBound < 0)
+                    {
+                        hasNegativeBucketBounds = true;
+                    }
+
                     totalCount += histogramMeasurement.BucketCount;
 
                     cursor = WriteMetricName(buffer, cursor, prometheusMetric, openMetricsRequested);
@@ -120,27 +126,32 @@ public static int WriteMetric(
                     previousBound = histogramMeasurement.ExplicitBound;
                 }
 
-                // Histogram sum
-                cursor = WriteMetricName(buffer, cursor, prometheusMetric, openMetricsRequested);
-                cursor = WriteAsciiStringNoEscape(buffer, cursor, "_sum");
-                cursor = WriteTags(buffer, cursor, metric, metricPoint.Tags, openMetricsRequested);
+                if (!openMetricsRequested || !hasNegativeBucketBounds)
+                {
+                    // OpenMetrics histograms with negative bucket thresholds MUST NOT expose
+                    // _sum and therefore MUST NOT expose _count.
+                    // See https://prometheus.io/docs/specs/om/open_metrics_spec/#histogram-1
+                    cursor = WriteMetricName(buffer, cursor, prometheusMetric, openMetricsRequested);
+                    cursor = WriteAsciiStringNoEscape(buffer, cursor, "_sum");
+                    cursor = WriteTags(buffer, cursor, metric, metricPoint.Tags, openMetricsRequested);
 
-                buffer[cursor++] = unchecked((byte)' ');
+                    buffer[cursor++] = unchecked((byte)' ');
 
-                cursor = WriteDouble(buffer, cursor, metricPoint.GetHistogramSum());
+                    cursor = WriteDouble(buffer, cursor, metricPoint.GetHistogramSum());
 
-                buffer[cursor++] = ASCII_LINEFEED;
+                    buffer[cursor++] = ASCII_LINEFEED;
 
-                // Histogram count
-                cursor = WriteMetricName(buffer, cursor, prometheusMetric, openMetricsRequested);
-                cursor = WriteAsciiStringNoEscape(buffer, cursor, "_count");
-                cursor = WriteTags(buffer, cursor, metric, metricPoint.Tags, openMetricsRequested);
+                    // Histogram count
+                    cursor = WriteMetricName(buffer, cursor, prometheusMetric, openMetricsRequested);
+                    cursor = WriteAsciiStringNoEscape(buffer, cursor, "_count");
+                    cursor = WriteTags(buffer, cursor, metric, metricPoint.Tags, openMetricsRequested);
 
-                buffer[cursor++] = unchecked((byte)' ');
+                    buffer[cursor++] = unchecked((byte)' ');
 
-                cursor = WriteLong(buffer, cursor, metricPoint.GetHistogramCount());
+                    cursor = WriteLong(buffer, cursor, metricPoint.GetHistogramCount());
 
-                buffer[cursor++] = ASCII_LINEFEED;
+                    buffer[cursor++] = ASCII_LINEFEED;
+                }
             }
         }
 

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs

@@ -997,6 +997,35 @@ public void HistogramOneDimensionWithScopeVersion()
             Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
+    [Fact]
+    public void HistogramWithNegativeBucketBoundsOmitsSumAndCountWithOpenMetricsFormat()
+    {
+        var buffer = new byte[85000];
+        var metrics = new List<Metric>();
+
+        using var meter = new Meter(Utils.GetCurrentMethodName());
+        using var provider = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddView(instrument => new ExplicitBucketHistogramConfiguration { Boundaries = [-1, 0, 1] })
+            .AddInMemoryExporter(metrics)
+            .Build();
+
+        var histogram = meter.CreateHistogram<double>("test_histogram");
+        histogram.Record(-0.5, new KeyValuePair<string, object?>("x", "1"));
+
+        provider.ForceFlush();
+
+        var cursor = WriteMetric(buffer, 0, metrics[0], true);
+        var output = Encoding.UTF8.GetString(buffer, 0, cursor);
+
+        Assert.Contains($"test_histogram_bucket{{otel_scope_name=\"{Utils.GetCurrentMethodName()}\",x=\"1\",le=\"-1\"}} 0\n", output, StringComparison.Ordinal);
+        Assert.Contains($"test_histogram_bucket{{otel_scope_name=\"{Utils.GetCurrentMethodName()}\",x=\"1\",le=\"0\"}} 1\n", output, StringComparison.Ordinal);
+        Assert.Contains($"test_histogram_bucket{{otel_scope_name=\"{Utils.GetCurrentMethodName()}\",x=\"1\",le=\"1\"}} 1\n", output, StringComparison.Ordinal);
+        Assert.Contains($"test_histogram_bucket{{otel_scope_name=\"{Utils.GetCurrentMethodName()}\",x=\"1\",le=\"+Inf\"}} 1\n", output, StringComparison.Ordinal);
+        Assert.DoesNotContain("test_histogram_sum{", output, StringComparison.Ordinal);
+        Assert.DoesNotContain("test_histogram_count{", output, StringComparison.Ordinal);
+    }
+
     [Fact]
     public void WriteAsciiStringNoEscapeWritesAsciiBytes()
     {