[Exporter.Prometheus] Fix spec-compliance issues

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

@@ -13,6 +13,15 @@ Notes](../../RELEASENOTES.md).
 * The library is now marked as trim and AOT compatible.
   ([#7441](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7441))
 
+* Fix double unit suffixes in metric names when using OpenMetrics.
+  ([#7454](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7454))
+
+* Fix incorrect handling of leading digits in metric names for OpenMetrics.
+  ([#7454](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7454))
+
+* Fix incorrect `Content-Type` for PrometheusText exposition formats.
+  ([#7454](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7454))
+
 ## 1.16.0-beta.1
 
 Released 2026-Jun-10

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

@@ -16,6 +16,15 @@ Notes](../../RELEASENOTES.md).
 * Removed the `PrometheusHttpListenerOptions.UriPrefixes` option.
   ([#7435](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7435))
 
+* Fix double unit suffixes in metric names when using OpenMetrics.
+  ([#7454](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7454))
+
+* Fix incorrect handling of leading digits in metric names for OpenMetrics.
+  ([#7454](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7454))
+
+* Fix incorrect `Content-Type` for PrometheusText exposition formats.
+  ([#7454](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7454))
+
 ## 1.16.0-beta.1
 
 Released 2026-Jun-10

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusMetric.cs

@@ -28,9 +28,16 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
             // [OpenMetrics UNIT metadata](https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#metricfamily)
             // and as a suffix to the metric name. The unit suffix comes before any type-specific suffixes.
             // https://github.com/open-telemetry/opentelemetry-specification/blob/3dfb383fe583e3b74a2365c5a1d90256b273ee76/specification/compatibility/prometheus_and_openmetrics.md#metric-metadata-1
+            // Each name is checked independently: openMetricsName has the _total counter suffix stripped
+            // (by RemoveOpenMetricsCounterNameSuffix above), so it may already end with the unit even
+            // when sanitizedName (which still carries _total) does not.
             if (!sanitizedName.EndsWith(sanitizedUnit, StringComparison.Ordinal))
             {
                 sanitizedName += $"_{sanitizedUnit}";
+            }
+
+            if (!openMetricsName.EndsWith(sanitizedUnit, StringComparison.Ordinal))
+            {
                 openMetricsName += $"_{sanitizedUnit}";
             }
         }
@@ -189,6 +196,7 @@ internal static string EscapeOpenMetricsName(string metricName)
                 sb ??= CreateStringBuilder(metricName);
                 sb.Append('_');
                 lastCharUnderscore = true;
+                continue;
             }
 
             if (!char.IsAsciiLetterOrDigit(c) && c != ':')

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusProtocol.cs

@@ -70,8 +70,15 @@ public static string GetContentType(PrometheusProtocol protocol)
         var builder = new StringBuilder()
             .Append(protocol.MediaType)
             .Append("; version=")
-            .Append(protocol.Version.ToString(3))
-            .Append("; charset=utf-8");
+            .Append(protocol.Version.ToString(3));
+
+        // The charset=utf-8 parameter is required by the OpenMetrics specification
+        // (https://prometheus.io/docs/specs/om/open_metrics_spec/#overall-structure)
+        // but is not part of the Prometheus text format specification.
+        if (protocol.IsOpenMetrics)
+        {
+            builder.Append("; charset=utf-8");
+        }
 
         if (protocol.Escaping is not null)
         {

test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs

@@ -547,7 +547,7 @@ private static async Task VerifyAsync(
         contentType ??=
             requestOpenMetrics ?
             "application/openmetrics-text; version=0.0.1; charset=utf-8" :
-            "text/plain; version=0.0.4; charset=utf-8";
+            "text/plain; version=0.0.4";
 
         Assert.Equal(contentType, response.Content.Headers.ContentType!.ToString());
         Assert.Equal(["Accept-Encoding"], response.Headers.Vary);

test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusIntegrationTests.cs

@@ -109,7 +109,7 @@ public async Task Scrape_Endpoint_Uses_GZip_When_Requested()
         Assert.Equal<string>(["gzip"], response.Content.Headers.ContentEncoding);
         Assert.Equal(["Accept-Encoding"], response.Headers.Vary);
         Assert.NotNull(response.Content.Headers.ContentType);
-        Assert.Equal("text/plain; version=0.0.4; charset=utf-8", response.Content.Headers.ContentType.ToString());
+        Assert.Equal("text/plain; version=0.0.4", response.Content.Headers.ContentType.ToString());
 
         using var compressed = await response.Content.ReadAsStreamAsync();
         using var decompressed = new GZipStream(compressed, CompressionMode.Decompress);

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

@@ -517,7 +517,7 @@ private static async Task RunPrometheusExporterHttpServerIntegrationTest(
             contentType ??=
                 requestOpenMetrics ?
                 "application/openmetrics-text; version=0.0.1; charset=utf-8" :
-                "text/plain; version=0.0.4; charset=utf-8";
+                "text/plain; version=0.0.4";
 
             Assert.NotNull(response.Content);
             Assert.NotNull(response.Content.Headers.ContentType);

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

@@ -164,8 +164,16 @@ public void OpenMetricsName_CollapsesConsecutiveUnderscores()
         => AssertOpenMetricsName("cpu_sp__d_hertz", string.Empty, PrometheusType.Gauge, false, "cpu_sp_d_hertz");
 
     [Fact]
-    public void OpenMetricsName_PreserveLeadingNumber()
-        => AssertOpenMetricsName("2_metric_name", "By", PrometheusType.Gauge, false, "_2_metric_name_bytes");
+    public void OpenMetricsName_ReplacesLeadingNumber()
+        => AssertOpenMetricsName("2_metric_name", "By", PrometheusType.Gauge, false, "_metric_name_bytes");
+
+    [Fact]
+    public void OpenMetricsName_CounterWithUnitPrecedingTotal_UnitNotDuplicated()
+        => AssertOpenMetricsName("db_bytes_total", "By", PrometheusType.Counter, false, "db_bytes_total");
+
+    [Fact]
+    public void OpenMetricsMetadataName_CounterWithUnitPrecedingTotal_UnitNotDuplicated()
+        => AssertOpenMetricsMetadataName("db_bytes_total", "By", PrometheusType.Counter, false, "db_bytes");
 
     [Fact]
     public void OpenMetricsName_UnitStartingWithNumber_DoesNotAddExtraSeparator()

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

@@ -711,4 +711,68 @@ public void Equals_Object_BoxingScenario()
         Assert.True(boxed1.Equals(boxed2));
         Assert.True(boxed2.Equals(boxed1));
     }
+
+    [Fact]
+    public void GetContentType_PrometheusTextV0_DoesNotIncludeCharset()
+    {
+        var protocol = new PrometheusProtocol(
+            PrometheusProtocol.PrometheusTextMediaType,
+            null,
+            PrometheusProtocol.PrometheusV0,
+            false);
+
+        Assert.Equal("text/plain; version=0.0.4", PrometheusProtocol.GetContentType(protocol));
+    }
+
+    [Fact]
+    public void GetContentType_PrometheusTextV1_DoesNotIncludeCharset()
+    {
+        var protocol = new PrometheusProtocol(
+            PrometheusProtocol.PrometheusTextMediaType,
+            PrometheusProtocol.UnderscoresEscaping,
+            PrometheusProtocol.PrometheusV1,
+            false);
+
+        Assert.Equal("text/plain; version=1.0.0; escaping=underscores", PrometheusProtocol.GetContentType(protocol));
+    }
+
+    [Fact]
+    public void GetContentType_OpenMetricsV0_IncludesCharset()
+    {
+        var protocol = new PrometheusProtocol(
+            PrometheusProtocol.OpenMetricsMediaType,
+            null,
+            PrometheusProtocol.OpenMetricsV0,
+            true);
+
+        Assert.Equal("application/openmetrics-text; version=0.0.1; charset=utf-8", PrometheusProtocol.GetContentType(protocol));
+    }
+
+    [Fact]
+    public void GetContentType_OpenMetricsV1_IncludesCharset()
+    {
+        var protocol = new PrometheusProtocol(
+            PrometheusProtocol.OpenMetricsMediaType,
+            PrometheusProtocol.UnderscoresEscaping,
+            PrometheusProtocol.OpenMetricsV1,
+            true);
+
+        Assert.Equal("application/openmetrics-text; version=1.0.0; charset=utf-8; escaping=underscores", PrometheusProtocol.GetContentType(protocol));
+    }
+
+    [Fact]
+    public void GetContentType_FallbackProtocol_DoesNotIncludeCharset() =>
+        Assert.Equal("text/plain; version=0.0.4", PrometheusProtocol.GetContentType(PrometheusProtocol.Fallback));
+
+    [Fact]
+    public void GetContentType_ToStringDelegatesToGetContentType()
+    {
+        var protocol = new PrometheusProtocol(
+            PrometheusProtocol.OpenMetricsMediaType,
+            null,
+            PrometheusProtocol.OpenMetricsV1,
+            true);
+
+        Assert.Equal(PrometheusProtocol.GetContentType(protocol), protocol.ToString());
+    }
 }