[Exporter.Prometheus] Update Accept/Content-Type handling to consider escaping

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

@@ -37,6 +37,15 @@ Notes](../../RELEASENOTES.md).
 * Fix incorrect handling of untyped metrics when using OpenMetrics format.
   ([#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.
+  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))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

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

@@ -15,9 +15,10 @@ namespace OpenTelemetry.Exporter;
 /// </summary>
 internal sealed class PrometheusExporterMiddleware
 {
+    private const string OpenMetricsEscapingScheme = "underscores";
     private const string OpenMetricsMediaType = "application/openmetrics-text";
     private const string OpenMetricsVersion = "1.0.0";
-    private const string OpenMetricsContentType = $"application/openmetrics-text; version={OpenMetricsVersion}; charset=utf-8";
+    private const string OpenMetricsContentType = $"application/openmetrics-text; version={OpenMetricsVersion}; charset=utf-8; escaping={OpenMetricsEscapingScheme}";
 
     private const string PrometheusTextMediaType = "text/plain";
 
@@ -123,7 +124,7 @@ internal static bool AcceptsOpenMetrics(HttpRequest request)
             }
 
             if (string.Equals(mediaType.MediaType.Value, OpenMetricsMediaType, StringComparison.OrdinalIgnoreCase) &&
-                HasSupportedOpenMetricsVersion(mediaType))
+                HasSupportedOpenMetricsParameters(mediaType))
             {
                 bestOpenMetricsQuality =
                     bestOpenMetricsQuality is not { } comparison || quality > comparison ?
@@ -143,16 +144,23 @@ bestPrometheusQuality is not { } comparison || quality > comparison ?
                (bestPrometheusQuality is not { } prometheusQuality || openMetricsQuality >= prometheusQuality);
     }
 
-    private static bool HasSupportedOpenMetricsVersion(MediaTypeHeaderValue value)
+    private static bool HasSupportedOpenMetricsParameters(MediaTypeHeaderValue value)
     {
+        var hasSupportedOpenMetricsEscaping = true;
+        var hasSupportedOpenMetricsVersion = true;
+
         foreach (var parameter in value.Parameters)
         {
             if (string.Equals(parameter.Name.Value, "version", StringComparison.OrdinalIgnoreCase))
             {
-                return string.Equals(parameter.Value.Value?.Trim('"'), OpenMetricsVersion, StringComparison.Ordinal);
+                hasSupportedOpenMetricsVersion = string.Equals(parameter.Value.Value?.Trim('"'), OpenMetricsVersion, StringComparison.Ordinal);
+            }
+            else if (string.Equals(parameter.Name.Value, "escaping", StringComparison.OrdinalIgnoreCase))
+            {
+                hasSupportedOpenMetricsEscaping = string.Equals(parameter.Value.Value?.Trim('"'), OpenMetricsEscapingScheme, StringComparison.Ordinal);
             }
         }
 
-        return true;
+        return hasSupportedOpenMetricsVersion && hasSupportedOpenMetricsEscaping;
     }
 }

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

@@ -54,6 +54,15 @@ Notes](../../RELEASENOTES.md).
   environment variables.
   ([#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.
+  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))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

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

@@ -238,7 +238,7 @@ private ExportResult OnCollect(in Batch<Metric> metrics)
                         {
                             try
                             {
-                                cursor = PrometheusSerializer.WriteScopeInfo(buffer, cursor, metric.MeterName);
+                                cursor = PrometheusSerializer.WriteScopeInfo(buffer, cursor, metric.MeterName, openMetricsRequested: true);
 
                                 break;
                             }
@@ -342,7 +342,7 @@ private int WriteTargetInfo(ref byte[] buffer)
             {
                 try
                 {
-                    this.targetInfoBufferLength = PrometheusSerializer.WriteTargetInfo(buffer, 0, this.exporter.Resource);
+                    this.targetInfoBufferLength = PrometheusSerializer.WriteTargetInfo(buffer, 0, this.exporter.Resource, openMetricsRequested: true);
 
                     break;
                 }

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

@@ -7,6 +7,7 @@ namespace OpenTelemetry.Exporter.Prometheus;
 
 internal static class PrometheusHeadersParser
 {
+    private const string OpenMetricsEscapingScheme = "underscores";
     private const string OpenMetricsMediaType = "application/openmetrics-text";
     private const string OpenMetricsVersion = "1.0.0";
     private const string PrometheusTextMediaType = "text/plain";
@@ -23,6 +24,7 @@ internal static bool AcceptsOpenMetrics(string? contentType)
             var mediaType = TrimWhitespace(SplitNext(ref headerValue, ';'));
             var quality = 1.0;
             var hasValidQuality = true;
+            var hasSupportedOpenMetricsEscaping = true;
             var hasSupportedOpenMetricsVersion = true;
 
             while (headerValue.Length > 0)
@@ -35,6 +37,10 @@ internal static bool AcceptsOpenMetrics(string? contentType)
                     {
                         hasSupportedOpenMetricsVersion = IsSupportedOpenMetricsVersion(parameter.Slice("version=".Length));
                     }
+                    else if (parameter.StartsWith("escaping=".AsSpan(), StringComparison.OrdinalIgnoreCase))
+                    {
+                        hasSupportedOpenMetricsEscaping = IsSupportedOpenMetricsEscaping(parameter.Slice("escaping=".Length));
+                    }
 
                     continue;
                 }
@@ -59,7 +65,9 @@ internal static bool AcceptsOpenMetrics(string? contentType)
                 continue;
             }
 
-            if (mediaType.Equals(OpenMetricsMediaType.AsSpan(), StringComparison.OrdinalIgnoreCase) && hasSupportedOpenMetricsVersion)
+            if (mediaType.Equals(OpenMetricsMediaType.AsSpan(), StringComparison.OrdinalIgnoreCase) &&
+                hasSupportedOpenMetricsVersion &&
+                hasSupportedOpenMetricsEscaping)
             {
                 bestOpenMetricsQuality =
                     bestOpenMetricsQuality is not { } comparison || quality > comparison ?
@@ -82,6 +90,9 @@ bestPrometheusQuality is not { } comparison || quality > comparison ?
     private static bool IsSupportedOpenMetricsVersion(ReadOnlySpan<char> value)
         => TrimQuotes(value).Equals(OpenMetricsVersion.AsSpan(), StringComparison.Ordinal);
 
+    private static bool IsSupportedOpenMetricsEscaping(ReadOnlySpan<char> value)
+        => TrimQuotes(value).Equals(OpenMetricsEscapingScheme.AsSpan(), StringComparison.Ordinal);
+
     private static ReadOnlySpan<char> SplitNext(ref ReadOnlySpan<char> span, char character)
     {
         var index = span.IndexOf(character);

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

@@ -17,7 +17,7 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
         // consecutive `_` characters MUST be replaced with a single `_` character.
         // https://github.com/open-telemetry/opentelemetry-specification/blob/b2f923fb1650dde1f061507908b834035506a796/specification/compatibility/prometheus_and_openmetrics.md#L230-L233
         var sanitizedName = SanitizeMetricName(name);
-        var openMetricsName = SanitizeOpenMetricsName(sanitizedName);
+        var openMetricsName = RemoveOpenMetricsCounterNameSuffix(name);
 
         string? sanitizedUnit = null;
         if (!string.IsNullOrEmpty(unit))
@@ -35,6 +35,8 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
             }
         }
 
+        openMetricsName = EscapeOpenMetricsName(openMetricsName);
+
         // If the metric name for monotonic Sum metric points does not end in a suffix of `_total` a suffix of `_total` MUST be added by default, otherwise the name MUST remain unchanged.
         // Exporters SHOULD provide a configuration option to disable the addition of `_total` suffixes.
         // https://github.com/open-telemetry/opentelemetry-specification/blob/b2f923fb1650dde1f061507908b834035506a796/specification/compatibility/prometheus_and_openmetrics.md#L286
@@ -54,8 +56,8 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
         // In OpenMetrics format, the UNIT, TYPE and HELP metadata must be suffixed with the unit (handled above), and not the '_total' suffix, as in the case for counters.
         // https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#unit
         var openMetricsMetadataName = type == PrometheusType.Counter
-            ? SanitizeOpenMetricsName(openMetricsName)
-            : sanitizedName;
+            ? RemoveOpenMetricsCounterNameSuffix(openMetricsName)
+            : openMetricsName;
 
         this.Name = sanitizedName;
         this.OpenMetricsName = openMetricsName;
@@ -148,6 +150,47 @@ static StringBuilder CreateStringBuilder(string value)
         }
     }
 
+    internal static string EscapeOpenMetricsName(string metricName)
+    {
+        StringBuilder? sb = null;
+        var lastCharUnderscore = false;
+
+        for (var i = 0; i < metricName.Length; i++)
+        {
+            var c = metricName[i];
+
+            if (i == 0 && char.IsAsciiDigit(c))
+            {
+                sb ??= CreateStringBuilder(metricName);
+                sb.Append('_');
+                lastCharUnderscore = true;
+            }
+
+            if (!char.IsAsciiLetterOrDigit(c) && c != ':')
+            {
+                if (!lastCharUnderscore)
+                {
+                    sb ??= CreateStringBuilder(metricName);
+                    sb.Append('_');
+                    lastCharUnderscore = true;
+                }
+            }
+            else
+            {
+                sb ??= CreateStringBuilder(metricName);
+                sb.Append(c);
+                lastCharUnderscore = c == '_';
+            }
+        }
+
+        return sb?.ToString() ?? metricName;
+
+        static StringBuilder CreateStringBuilder(string value)
+        {
+            return new(value.Length + 1);
+        }
+    }
+
     internal static string RemoveAnnotations(string unit)
     {
         // UCUM standard says the curly braces shouldn't be nested:
@@ -215,7 +258,7 @@ UpDownCounter becomes gauge
         };
     }
 
-    private static string SanitizeOpenMetricsName(string metricName)
+    private static string RemoveOpenMetricsCounterNameSuffix(string metricName)
         => metricName.EndsWith("_total", StringComparison.Ordinal) ? metricName.Substring(0, metricName.Length - 6) : metricName;
 
     private static string GetUnit(string unit)