[Prometheus.AspNetCore] Support GZip

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

@@ -80,6 +80,10 @@ Notes](../../RELEASENOTES.md).
   OpenMetrics and a start time is available.
   ([#7223](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7223))
 
+* GZip compress scrape endpoint responses when `Accept-Encoding: gzip` is
+  specified by the HTTP request headers.
+  ([#7274](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7274))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/OpenTelemetry.Exporter.Prometheus.AspNetCore.csproj

@@ -7,6 +7,7 @@
     <PackageTags>$(PackageTags);prometheus;metrics</PackageTags>
     <MinVerTagPrefix>coreunstable-</MinVerTagPrefix>
     <DefineConstants>$(DefineConstants);PROMETHEUS_ASPNETCORE</DefineConstants>
+    <NoWarn>$(NoWarn);CA2007</NoWarn>
   </PropertyGroup>
 
   <ItemGroup Condition="'$(RunningDotNetPack)' != 'true'">

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

@@ -5,7 +5,9 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
+using System.IO.Compression;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Headers;
 using Microsoft.Net.Http.Headers;
 using OpenTelemetry.Exporter.Prometheus;
 using OpenTelemetry.Internal;
@@ -73,9 +75,11 @@ public async Task InvokeAsync(HttpContext httpContext)
 
             using var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(requestCancelled.Token, httpContext.RequestAborted);
 
-            var protocol = Negotiate(httpContext.Request);
+            var requestHeaders = httpContext.Request.GetTypedHeaders();
 
-            var collectionResponse = await this.exporter.CollectionManager.EnterCollect(protocol.IsOpenMetrics).ConfigureAwait(false);
+            var protocol = Negotiate(requestHeaders);
+
+            var collectionResponse = await this.exporter.CollectionManager.EnterCollect(protocol.IsOpenMetrics);
 
             try
             {
@@ -90,7 +94,7 @@ public async Task InvokeAsync(HttpContext httpContext)
                     response.Headers.Append("Last-Modified", collectionResponse.GeneratedAtUtc.ToString("R"));
                     response.ContentType = PrometheusProtocol.GetContentType(protocol);
 
-                    await response.Body.WriteAsync(dataView.Array.AsMemory(0, dataView.Count), linkedCts.Token).ConfigureAwait(false);
+                    await WriteResponseAsync(response, dataView.Array.AsMemory(0, dataView.Count), AcceptsGZip(requestHeaders), linkedCts.Token);
                 }
                 else
                 {
@@ -144,9 +148,9 @@ static bool TryGetScrapeTimeout(
         }
     }
 
-    internal static PrometheusProtocol Negotiate(HttpRequest request)
+    internal static PrometheusProtocol Negotiate(RequestHeaders headers)
     {
-        var acceptHeader = request.GetTypedHeaders().Accept;
+        var acceptHeader = headers.Accept;
 
         if (acceptHeader is not { Count: > 0 })
         {
@@ -284,4 +288,46 @@ private static bool TryParse(
         protocol = new(mediaType, escaping, version, isOpenMetrics);
         return true;
     }
+
+    private static bool AcceptsGZip(RequestHeaders headers)
+    {
+        if (headers.AcceptEncoding is { Count: > 0 } acceptEncoding)
+        {
+            foreach (var parameter in acceptEncoding)
+            {
+                if (parameter.Quality is not 0 && parameter.Value.Equals("gzip", StringComparison.OrdinalIgnoreCase))
+                {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+
+    private static async Task WriteResponseAsync(
+        HttpResponse response,
+        ReadOnlyMemory<byte> content,
+        bool compress,
+        CancellationToken cancellationToken)
+    {
+        response.Headers.AppendCommaSeparatedValues(HeaderNames.Vary, HeaderNames.AcceptEncoding);
+
+        if (compress)
+        {
+            response.Headers.Append(HeaderNames.ContentEncoding, "gzip");
+
+            await using var gzip = new GZipStream(
+                response.Body,
+                CompressionLevel.Fastest,
+                leaveOpen: true);
+
+            await gzip.WriteAsync(content, cancellationToken);
+            await gzip.FlushAsync(cancellationToken);
+        }
+        else
+        {
+            await response.BodyWriter.WriteAsync(content, cancellationToken);
+        }
+    }
 }

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

@@ -237,7 +237,7 @@ public void PrometheusExporterMiddlewareNegotiate_UsesTypedAcceptHeaders(
         var context = new DefaultHttpContext();
         context.Request.Headers.Accept = accept;
 
-        var actual = PrometheusExporterMiddleware.Negotiate(context.Request);
+        var actual = PrometheusExporterMiddleware.Negotiate(context.Request.GetTypedHeaders());
 
         Assert.Equal(mediaType, actual.MediaType);
         Assert.Equal(isOpenMetrics, actual.IsOpenMetrics);
@@ -252,7 +252,7 @@ public void PrometheusExporterMiddlewareNegotiate_UsesFallbackForInvalidHeader(s
         var context = new DefaultHttpContext();
         context.Request.Headers.Accept = accept;
 
-        var actual = PrometheusExporterMiddleware.Negotiate(context.Request);
+        var actual = PrometheusExporterMiddleware.Negotiate(context.Request.GetTypedHeaders());
 
         Assert.Equivalent(PrometheusProtocol.Fallback, actual);
     }
@@ -552,6 +552,7 @@ private static async Task VerifyAsync(
             "text/plain; version=0.0.4; charset=utf-8";
 
         Assert.Equal(contentType, response.Content.Headers.ContentType!.ToString());
+        Assert.Equal(["Accept-Encoding"], response.Headers.Vary);
 
         var additionalTags = meterTags is { Length: > 0 }
             ? $"{string.Join(",", meterTags.Select(x => $"{x.Key}=\"{x.Value}\""))},"

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

@@ -3,6 +3,7 @@
 
 using System.Diagnostics;
 using System.Diagnostics.Metrics;
+using System.IO.Compression;
 using System.Net;
 using System.Net.Http.Json;
 using System.Text.Json;
@@ -71,13 +72,63 @@ public async Task Scrape_Endpoint_Returns_No_Content_If_Sdk_Disabled()
         Assert.Empty(content);
     }
 
+    [Fact]
+    public async Task Scrape_Endpoint_Uses_GZip_When_Requested()
+    {
+        // Arrange
+        var builder = WebApplication.CreateBuilder();
+
+        // Listen on any available port
+        builder.WebHost.UseUrls("http://127.0.0.1:0");
+
+        builder.Services
+            .AddOpenTelemetry()
+            .WithMetrics((builder) => builder.AddPrometheusExporter());
+
+        using var app = builder.Build();
+
+        app.MapPrometheusScrapingEndpoint();
+
+        await app.StartAsync();
+
+        var server = app.Services.GetRequiredService<IServer>();
+        var addresses = server.Features.Get<IServerAddressesFeature>();
+
+        var baseAddress = addresses!.Addresses
+            .Select((p) => new Uri(p))
+            .Last();
+
+        using var httpClient = new HttpClient();
+
+        httpClient.DefaultRequestHeaders.Add("Accept-Encoding", "gzip");
+
+        using var response = await httpClient.GetAsync(new Uri(baseAddress, "metrics"));
+
+        // Assert
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.NotNull(response.Content);
+        Assert.NotNull(response.Content.Headers.ContentEncoding);
+        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());
+
+        using var compressed = await response.Content.ReadAsStreamAsync();
+        using var decompressed = new GZipStream(compressed, CompressionMode.Decompress);
+        using var reader = new StreamReader(decompressed);
+
+        var content = await reader.ReadToEndAsync();
+
+        Assert.NotEmpty(content);
+        Assert.EndsWith("# EOF\n", content, StringComparison.Ordinal);
+    }
+
     [EnabledOnDockerPlatformTheory(DockerPlatform.Linux)]
     [InlineData("")]
     [InlineData("OpenMetricsText0.0.1")]
     [InlineData("OpenMetricsText1.0.0")]
     [InlineData("PrometheusText0.0.4")]
     [InlineData("PrometheusText1.0.0")]
-
     public async Task Prometheus_Can_Scrape_Metrics(string scrapeProtocol) => await GenerateMetricsAsync(async (baseAddress) =>
     {
         // Arrange