Add histogram exemplar support to Prometheus exporter

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

@@ -9,6 +9,7 @@ Notes](../../RELEASENOTES.md).
 
 * Added meter-level tags to Prometheus exporter
   ([#5837](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5837))
+* Added histogram exemplar support to Prometheus exporter
 
 ## 1.9.0-beta.2
 

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

@@ -18,14 +18,6 @@ to scrape.
   Grafana](../../docs/metrics/getting-started-prometheus-grafana/README.md)
   tutorial for more information.
 
-<!-- This comment is to make sure the two notes above and below are not merged -->
-
-> [!NOTE]
-> This exporter does not support Exemplars. For using Exemplars, use the [OTLP
-Exporter](../OpenTelemetry.Exporter.OpenTelemetryProtocol/README.md) and use a
-component like OTel Collector to expose metrics (with exemplars) to Prometheus.
-This [tutorial](../../docs/metrics/exemplars/README.md) shows one way how to do that.
-
 ## Prerequisite
 
 * [Get Prometheus](https://prometheus.io/docs/introduction/first_steps/)

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

@@ -9,6 +9,7 @@ Notes](../../RELEASENOTES.md).
 
 * Added meter-level tags to Prometheus exporter
   ([#5837](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5837))
+* Added histogram exemplar support to Prometheus exporter
 
 ## 1.9.0-beta.2
 

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

@@ -64,4 +64,10 @@ public void NoMetrics()
     {
         this.WriteEvent(4);
     }
+
+    [Event(5, Message = "Ignoring exemplar tags that are too long for metric: '{0}'", Level = EventLevel.Warning)]
+    public void ExemplarTagsTooLong(string metricName)
+    {
+        this.WriteEvent(5);
+    }
 }

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

@@ -80,6 +80,10 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric, Promethe
                 var tags = metricPoint.Tags;
                 var timestamp = metricPoint.EndTime.ToUnixTimeMilliseconds();
 
+                metricPoint.TryGetExemplars(out var exemplarCollection);
+                var exemplars = exemplarCollection.GetEnumerator();
+                var hasExemplar = exemplars.MoveNext();
+
                 long totalCount = 0;
                 foreach (var histogramMeasurement in metricPoint.GetHistogramBuckets())
                 {
@@ -107,6 +111,19 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric, Promethe
 
                     cursor = WriteTimestamp(buffer, cursor, timestamp, openMetricsRequested);
 
+                    if (hasExemplar)
+                    {
+                        if (exemplars.Current.DoubleValue <= histogramMeasurement.ExplicitBound)
+                        {
+                            cursor = WriteExemplar(buffer, cursor, exemplars.Current, openMetricsRequested, metric.Name);
+                        }
+
+                        while (hasExemplar && exemplars.Current.DoubleValue <= histogramMeasurement.ExplicitBound)
+                        {
+                            hasExemplar = exemplars.MoveNext();
+                        }
+                    }
+
                     buffer[cursor++] = ASCII_LINEFEED;
                 }
 
@@ -142,4 +159,55 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric, Promethe
 
         return cursor;
     }
+
+    private static int WriteExemplar(byte[] buffer, int cursor, in Exemplar exemplar, bool openMetricsRequested, string metricName)
+    {
+        buffer[cursor++] = unchecked((byte)' ');
+        buffer[cursor++] = unchecked((byte)'#');
+        buffer[cursor++] = unchecked((byte)' ');
+
+        buffer[cursor++] = unchecked((byte)'{');
+        var labelSetCursorStart = cursor;
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, "trace_id=\"");
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, exemplar.TraceId.ToHexString());
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, "\",span_id=\"");
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, exemplar.SpanId.ToHexString());
+        buffer[cursor++] = unchecked((byte)'"');
+        buffer[cursor++] = unchecked((byte)',');
+
+        var labelSetWritten = cursor - labelSetCursorStart - 8;
+
+        var tagResetCursor = cursor;
+
+        foreach (var tag in exemplar.FilteredTags)
+        {
+            var prevCursor = cursor;
+            cursor = WriteLabel(buffer, cursor, tag.Key, tag.Value);
+
+            // From the spec:
+            //   Other characters in the text rendering of an exemplar such as ",= are not included in this limit
+            //   for implementation simplicity and for consistency between the text and proto formats.
+            labelSetWritten += cursor - prevCursor - 3; // subtract 2 x " and 1 x = character
+
+            buffer[cursor++] = unchecked((byte)',');
+
+            // From the spec:
+            //   The combined length of the label names and values of an Exemplar's LabelSet MUST NOT exceed 128 UTF-8 character code points.
+            if (labelSetWritten > 128)
+            {
+                cursor = tagResetCursor;
+                PrometheusExporterEventSource.Log.ExemplarTagsTooLong(metricName);
+                break;
+            }
+        }
+
+        buffer[cursor - 1] = unchecked((byte)'}'); // Note: We write the '}' over the last written comma, which is extra.
+        buffer[cursor++] = unchecked((byte)' ');
+
+        cursor = WriteDouble(buffer, cursor, exemplar.DoubleValue);
+        buffer[cursor++] = unchecked((byte)' ');
+        cursor = WriteTimestamp(buffer, cursor, exemplar.Timestamp.ToUnixTimeMilliseconds(), openMetricsRequested);
+
+        return cursor;
+    }
 }