[Exporter.Prometheus] Fix untyped for OpenMetrics

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

@@ -34,6 +34,9 @@ Notes](../../RELEASENOTES.md).
 * Emit OpenMetrics exemplars for counters and histogram buckets.
   ([#7222](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7222))
 
+* Fix incorrect handling of untyped metrics when using OpenMetrics format.
+  ([#7219](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7219))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

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

@@ -46,6 +46,9 @@ Notes](../../RELEASENOTES.md).
 * Emit OpenMetrics exemplars for counters and histogram buckets.
   ([#7222](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7222))
 
+* Fix incorrect handling of untyped metrics when using OpenMetrics format.
+  ([#7219](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7219))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

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

@@ -389,7 +389,7 @@ public static int WriteHelpMetadata(byte[] buffer, int cursor, PrometheusMetric
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static int WriteTypeMetadata(byte[] buffer, int cursor, PrometheusMetric metric, bool openMetricsRequested)
     {
-        var metricType = MapPrometheusType(metric.Type);
+        var metricType = MapPrometheusType(metric.Type, openMetricsRequested);
 
         Debug.Assert(!string.IsNullOrEmpty(metricType), $"{nameof(metricType)} should not be null or empty.");
 
@@ -554,12 +554,15 @@ private static int WriteUnicodeScalar(byte[] buffer, int cursor, string value, r
     private static int WriteUnixTimeSeconds(byte[] buffer, int cursor, DateTimeOffset value)
         => WriteDouble(buffer, cursor, value.ToUnixTimeMilliseconds() / 1000.0);
 
-    private static string MapPrometheusType(PrometheusType type) => type switch
+    private static string MapPrometheusType(PrometheusType type, bool openMetricsRequested) => type switch
     {
         PrometheusType.Gauge => "gauge",
         PrometheusType.Counter => "counter",
         PrometheusType.Summary => "summary",
         PrometheusType.Histogram => "histogram",
-        PrometheusType.Untyped or _ => "untyped",
+
+        // OpenMetrics 1.0 uses "unknown" while Prometheus text format 0.0.4 uses "untyped".
+        // See https://prometheus.io/docs/specs/om/open_metrics_spec/#unknown-1
+        PrometheusType.Untyped or _ => openMetricsRequested ? "unknown" : "untyped",
     };
 }

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

@@ -956,6 +956,28 @@ public void WriteAsciiStringNoEscapeWritesAsciiBytes()
         Assert.Equal("metric_name_total", Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
+    [Fact]
+    public void UntypedMetricUsesUnknownTypeForOpenMetrics()
+    {
+        var buffer = new byte[64];
+        var metric = new PrometheusMetric("test_metric", string.Empty, PrometheusType.Untyped, disableTotalNameSuffixForCounters: false);
+
+        var cursor = PrometheusSerializer.WriteTypeMetadata(buffer, 0, metric, openMetricsRequested: true);
+
+        Assert.Equal("# TYPE test_metric unknown\n", Encoding.UTF8.GetString(buffer, 0, cursor));
+    }
+
+    [Fact]
+    public void UntypedMetricUsesUntypedTypeForPrometheusTextFormat()
+    {
+        var buffer = new byte[64];
+        var metric = new PrometheusMetric("test_metric", string.Empty, PrometheusType.Untyped, disableTotalNameSuffixForCounters: false);
+
+        var cursor = PrometheusSerializer.WriteTypeMetadata(buffer, 0, metric, openMetricsRequested: false);
+
+        Assert.Equal("# TYPE test_metric untyped\n", Encoding.UTF8.GetString(buffer, 0, cursor));
+    }
+
     [Fact]
     public void WriteAsciiStringNoEscapeThrowsExceptionWhenBufferTooSmall()
     {