[Exporter.Prometheus] Add dots and values escaping

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

@@ -23,6 +23,10 @@ Notes](../../RELEASENOTES.md).
   disable scope labels in Prometheus metrics. Defaults to `true`.
   ([#7436](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7436))
 
+* Added support for the `dots` and `values` Prometheus UTF-8 name escaping
+  schemes when negotiated via the `Accept` header.
+  ([#7439](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7439))
+
 ## 1.16.0-beta.1
 
 Released 2026-Jun-10

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterMiddleware.cs

@@ -273,6 +273,7 @@ private static bool TryParse(
         {
             // Use the oldest version if no version preference was specified
             version = isOpenMetrics ? PrometheusProtocol.OpenMetricsV0 : PrometheusProtocol.PrometheusV0;
+            escaping = null;
         }
         else if (version.Major is not > 0)
         {

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

@@ -26,6 +26,10 @@ Notes](../../RELEASENOTES.md).
   disable scope labels in Prometheus metrics. Defaults to `true`.
   ([#7436](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7436))
 
+* Added support for the `dots` and `values` Prometheus UTF-8 name escaping
+  schemes when negotiated via the `Accept` header.
+  ([#7439](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7439))
+
 ## 1.16.0-beta.1
 
 Released 2026-Jun-10

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

@@ -120,6 +120,7 @@ internal static PrometheusProtocol Negotiate(string? contentType)
             {
                 // Use the oldest version if no version preference was specified
                 version = isOpenMetrics ? PrometheusProtocol.OpenMetricsV0 : PrometheusProtocol.PrometheusV0;
+                escaping = null;
             }
             else if (version.Major is not > 0)
             {

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

@@ -0,0 +1,30 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+namespace OpenTelemetry.Exporter.Prometheus;
+
+/// <summary>
+/// The Prometheus UTF-8 name escaping scheme used when rendering metric and label names.
+/// See https://prometheus.io/docs/instrumenting/escaping_schemes/.
+/// </summary>
+internal enum EscapingScheme
+{
+    /// <summary>
+    /// Discouraged characters are replaced with the <c>_</c> character. This is the
+    /// OpenTelemetry default and also covers the legacy (pre-1.0.0) text formats which
+    /// have no negotiated escaping scheme.
+    /// </summary>
+    Underscores = 0,
+
+    /// <summary>
+    /// Dots are replaced with <c>_dot_</c> and underscores are doubled, allowing the
+    /// original name to be recovered by a Prometheus client.
+    /// </summary>
+    Dots,
+
+    /// <summary>
+    /// Names that are not valid legacy names are prefixed with <c>U__</c> and discouraged
+    /// characters are encoded as their hexadecimal Unicode code point.
+    /// </summary>
+    Values,
+}

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

@@ -42,9 +42,9 @@ public PrometheusCollectionManager(PrometheusExporter exporter)
     internal Func<TimeSpan> GetElapsedTime { get; set; }
 
 #if NET
-    public ValueTask<CollectionResponse> EnterCollect(PrometheusProtocol protocol)
+    public ValueTask<CollectionResponse> EnterCollect(in PrometheusProtocol protocol)
 #else
-    public Task<CollectionResponse> EnterCollect(PrometheusProtocol protocol)
+    public Task<CollectionResponse> EnterCollect(in PrometheusProtocol protocol)
 #endif
     {
         CollectionResponse? cachedResponse = null;
@@ -213,7 +213,7 @@ public Task<CollectionResponse> EnterCollect(PrometheusProtocol protocol)
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public void ExitCollect(PrometheusProtocol protocol)
+    public void ExitCollect(in PrometheusProtocol protocol)
         => this.GetProtocolState(protocol).DecrementReaderCount();
 
 #if NET
@@ -259,15 +259,15 @@ private void ExitGlobalLock()
         => Interlocked.Exchange(ref this.globalLockState, 0);
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private void IncrementReaderCount(PrometheusProtocol protocol)
+    private void IncrementReaderCount(in PrometheusProtocol protocol)
         => this.GetProtocolState(protocol).IncrementReaderCount();
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private bool HasActiveReaders(PrometheusProtocol protocol)
+    private bool HasActiveReaders(in PrometheusProtocol protocol)
         => this.protocolStates.TryGetValue(protocol, out var state) && state.HasActiveReaders();
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private bool WaitForReadersToComplete(PrometheusProtocol protocol)
+    private bool WaitForReadersToComplete(in PrometheusProtocol protocol)
     {
         var state = this.GetProtocolState(protocol);
         var didSpin = false;
@@ -347,7 +347,7 @@ private ExportResult OnCollect(in Batch<Metric> metrics)
             : ExportResult.Failure;
     }
 
-    private bool TryWriteResponse(PrometheusProtocol protocol, PrometheusProtocolState state, in Batch<Metric> metrics)
+    private bool TryWriteResponse(in PrometheusProtocol protocol, PrometheusProtocolState state, in Batch<Metric> metrics)
     {
         try
         {
@@ -455,7 +455,7 @@ private CollectionResult CreateCollectionResult(CollectionContext collectionCont
         return new CollectionResult(responses);
     }
 
-    private bool TryGetCachedResponse(PrometheusProtocol protocol, out CollectionResponse response)
+    private bool TryGetCachedResponse(in PrometheusProtocol protocol, out CollectionResponse response)
     {
         if (this.protocolStates.TryGetValue(protocol, out var state) &&
             state.GeneratedAt is { } generatedAt &&
@@ -472,7 +472,7 @@ state.GeneratedAtElapsed is { } generatedAtElapsed &&
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private PrometheusProtocolState GetProtocolState(PrometheusProtocol protocol)
+    private PrometheusProtocolState GetProtocolState(in PrometheusProtocol protocol)
         => this.protocolStates.GetOrAdd(protocol, static _ => new());
 
     private int WriteTargetInfo(TextFormatSerializer serializer, PrometheusProtocolState state)
@@ -637,7 +637,7 @@ public CollectionResult(IReadOnlyDictionary<PrometheusProtocol, CollectionRespon
             this.responses = responses;
         }
 
-        public bool TryGetResponse(PrometheusProtocol protocol, out CollectionResponse response)
+        public bool TryGetResponse(in PrometheusProtocol protocol, out CollectionResponse response)
         {
             if (this.responses?.TryGetValue(protocol, out response) == true)
             {
@@ -736,7 +736,7 @@ private sealed class CollectionContext
         private readonly TaskCompletionSource<CollectionResult> tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
         private bool frozen;
 
-        public CollectionContext(PrometheusProtocol protocol)
+        public CollectionContext(in PrometheusProtocol protocol)
         {
             this.protocols.Add(protocol);
         }
@@ -755,7 +755,7 @@ public PrometheusProtocol[] FreezeProtocols()
         public void SetResult(CollectionResult result)
             => this.tcs.SetResult(result);
 
-        public bool TryRegisterProtocol(PrometheusProtocol protocol, bool hasActiveReaders)
+        public bool TryRegisterProtocol(in PrometheusProtocol protocol, bool hasActiveReaders)
         {
             lock (this.gate)
             {