[Exporter.Prometheus] Fix Accept header handling by martincostello · Pull Request #7208 · open-telemetry/opentelemetry-dotnet

martincostello · 2026-04-29T10:03:26Z

Changes

Update the handling of the Accept header to follow the requirements documented in Scrape protocol content negotiation.

The behaviour isn't 100% compliant yet - it requires some tweaks to the serializer to be fully-compliant as I've not included it here and are in #7209.

Merge requirement checklist

CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
Unit tests added/updated
Appropriate CHANGELOG.md files updated for non-trivial changes
~~Changes in public API reviewed (if applicable)~~

- Update the handling of the `Accept` header to follow the requirements documented in https://prometheus.io/docs/instrumenting/content_negotiation/. - Removed unused code.

martincostello · 2026-04-29T10:04:19Z

    <Compile Include="$(RepoRoot)\src\OpenTelemetry.Exporter.Prometheus.HttpListener\Internal\PrometheusSerializerExt.cs" Link="Includes/PrometheusSerializerExt.cs" />
    <Compile Include="$(RepoRoot)\src\OpenTelemetry.Exporter.Prometheus.HttpListener\Internal\PrometheusType.cs" Link="Includes/PrometheusType.cs" />
    <Compile Include="$(RepoRoot)\src\OpenTelemetry.Exporter.Prometheus.HttpListener\Internal\PrometheusMetric.cs" Link="Includes/PrometheusMetric.cs" />
-    <Compile Include="$(RepoRoot)\src\OpenTelemetry.Exporter.Prometheus.HttpListener\Internal\PrometheusHeadersParser.cs" Link="Includes/PrometheusHeadersParser.cs" />


This was removed in favour of a native implementation as ASP.NET Core has built-in parsers for the Accept header and we were manually re-parsing it.

codecov · 2026-04-29T10:08:05Z

Codecov Report

❌ Patch coverage is 98.64865% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.68%. Comparing base (24832b1) to head (749cdab).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...s.HttpListener/Internal/PrometheusHeadersParser.cs	97.91%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7208   +/-   ##
=======================================
  Coverage   89.67%   89.68%           
=======================================
  Files         272      272           
  Lines       13378    13434   +56     
=======================================
+ Hits        11997    12048   +51     
- Misses       1381     1386    +5

Flag	Coverage Δ
unittests-Project-Experimental	`89.46% <98.64%> (-0.10%)`	⬇️
unittests-Project-Stable	`89.43% <98.64%> (-0.16%)`	⬇️
unittests-UnstableCoreLibraries-Experimental	`41.32% <98.64%> (+0.35%)`	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	`98.24% <100.00%> (+0.74%)`	⬆️
...s.HttpListener/Internal/PrometheusHeadersParser.cs	`98.14% <97.91%> (-1.86%)`	⬇️

... and 5 files with indirect coverage changes

- Add a constant for the OpenMetrics version. - Update CHANGELOGs with PR number.

Copilot

Pull request overview

Updates Prometheus exporter content negotiation to better follow Prometheus scrape protocol rules by improving Accept header parsing and OpenMetrics vs Prometheus text selection.

Changes:

Enhanced Accept header parsing to account for whitespace, q weights, and OpenMetrics version (including quoted values).
Updated ASP.NET Core middleware to use typed Accept headers (GetTypedHeaders().Accept) and select the preferred format by quality.
Expanded unit test coverage for header parsing/negotiation scenarios and updated changelogs.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHeadersParserTests.cs	Adds additional valid/invalid `Accept` header test cases (whitespace, quoted version, `q` weights).
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs	Refactors some tests and adds a focused unit test for `AcceptsOpenMetrics` using typed header parsing.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusHeadersParser.cs	Implements `Accept` parsing with whitespace trimming, version parsing (quoted), and `q`-based negotiation.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents the improved `Accept` header handling change.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterMiddleware.cs	Switches to typed `Accept` parsing and applies `q`-based OpenMetrics vs text selection.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/OpenTelemetry.Exporter.Prometheus.AspNetCore.csproj	Stops linking the shared `PrometheusHeadersParser` into the AspNetCore project.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Documents the improved `Accept` header handling change.

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- Parse `Accept` case-insensitively. - Check `quality` is between 0 and 1.

Updated [OpenTelemetry.Exporter.OpenTelemetryProtocol](https://github.com/open-telemetry/opentelemetry-dotnet) from 1.15.3 to 1.16.0. <details> <summary>Release notes</summary> _Sourced from [OpenTelemetry.Exporter.OpenTelemetryProtocol's releases](https://github.com/open-telemetry/opentelemetry-dotnet/releases)._ ## 1.16.0 For highlights and announcements pertaining to this release see: [Release Notes > 1.16.0](https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/RELEASENOTES.md#1160). The following changes are from the previous release [1.16.0-rc.1](https://github.com/open-telemetry/opentelemetry-dotnet/releases/tag/core-1.16.0-rc.1). * NuGet: [OpenTelemetry v1.16.0](https://www.nuget.org/packages/OpenTelemetry/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Api v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Api/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry.Api/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Api.ProviderBuilderExtensions v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Api.ProviderBuilderExtensions/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry.Api.ProviderBuilderExtensions/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Exporter.Console v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Exporter.Console/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry.Exporter.Console/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Exporter.InMemory v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Exporter.InMemory/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry.Exporter.InMemory/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Exporter.OpenTelemetryProtocol v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Exporter.OpenTelemetryProtocol/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Exporter.Zipkin v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Exporter.Zipkin/1.16.0) No notable changes. See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0/src/OpenTelemetry.Exporter.Zipkin/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Extensions.Hosting v1.16.0](https://www.nuget.org/packages/OpenTelemetry.Extensions.Hosting/1.16.0) No notable changes. ... (truncated) ## 1.16.0-rc.1 The following changes are from the previous release [1.15.3](https://github.com/open-telemetry/opentelemetry-dotnet/releases/tag/core-1.15.3). * NuGet: [OpenTelemetry v1.16.0-rc.1](https://www.nuget.org/packages/OpenTelemetry/1.16.0-rc.1) * Stop validating View-provided metric stream `Name` against the instrument name syntax, per [spec clarification](open-telemetry/opentelemetry-specification#5094). ([#7300](open-telemetry/opentelemetry-dotnet#7300)) * Fix incorrect validation of `OTEL_BSP_*` and `OTEL_BLRP_*` environment variables. ([#7187](open-telemetry/opentelemetry-dotnet#7187)) * Fix observable instrument callbacks running once per reader instead of once per collection cycle. ([#7188](open-telemetry/opentelemetry-dotnet#7188)) * Added exception safety for user-supplied `ExemplarReservoir` implementations. Exceptions thrown from `Offer` are now caught and logged rather than propagating out of `Counter.Add`/`Histogram.Record`. ([#7277](open-telemetry/opentelemetry-dotnet#7277)) * Update `OpenTelemetrySdkEventSource` to support the W3C randomness flag. ([#7301](open-telemetry/opentelemetry-dotnet#7301)) * Added `ObservedTimestamp` property to `LogRecord`. ([#6979](open-telemetry/opentelemetry-dotnet#6979)) * **Breaking Change** Explicit histogram boundaries no longer allow more than 10 million values. ([#7165](open-telemetry/opentelemetry-dotnet#7165)) * Fixed a circular reference which could cause a `LoggerProvider` to fail to resolve when one of its dependencies depends on `ILogger` or `ILoggerFactory`. As part of this fix the `LoggerProvider` resolved from dependency injection is now created lazily when the first logger is created rather than when `ILoggerProvider` or `ILoggerFactory` is resolved. A consequence is that any invalid configuration now surfaces when the first log record is written instead of when the logging services are resolved. ([#7308](open-telemetry/opentelemetry-dotnet#7308)) See [CHANGELOG](https://github.com/open-telemetry/opentelemetry-dotnet/blob/core-1.16.0-rc.1/src/OpenTelemetry/CHANGELOG.md) for details. * NuGet: [OpenTelemetry.Api v1.16.0-rc.1](https://www.nuget.org/packages/OpenTelemetry.Api/1.16.0-rc.1) * **Experimental (pre-release builds only):** Add support for using environment variables as context propagation carriers. ([#7174](open-telemetry/opentelemetry-dotnet#7174)) * Fix `BaggagePropagator` to correctly follow Key and Value Encoding rules as per ... (truncated) ## 1.16.0-beta.1 The following changes are from the previous release [1.15.3-beta.1](https://github.com/open-telemetry/opentelemetry-dotnet/releases/tag/coreunstable-1.15.3-beta.1). * NuGet: [OpenTelemetry.Exporter.Prometheus.AspNetCore v1.16.0-beta.1](https://www.nuget.org/packages/OpenTelemetry.Exporter.Prometheus.AspNetCore/1.16.0-beta.1) * Fixed scrape response cache freshness using monotonic time so it is not affected by NTP system clock adjustments. ([#7253](open-telemetry/opentelemetry-dotnet#7253)) * **Breaking Change** Removed `DisableTimestamp` property from `PrometheusAspNetCoreOptions`. ([#7176](open-telemetry/opentelemetry-dotnet#7176)) * Fixed the serialization of `NaN`, `PositiveInfinity`, and `NegativeInfinity` values in Prometheus metrics to be compliant with the specification. ([#7179](open-telemetry/opentelemetry-dotnet#7179)) * Fixed loss of precision when serializing `double` and `float` values in Prometheus metrics to be compliant with the specification by using 17 significant digits to represent such values. ([#7179](open-telemetry/opentelemetry-dotnet#7179)) * Fix non-ASCII characters in metric names and unit strings not being sanitized correctly during Prometheus serialization. ([#7184](open-telemetry/opentelemetry-dotnet#7184)) * Fix case where reader tracking could be reset while readers were still active. ([#7190](open-telemetry/opentelemetry-dotnet#7190)) * Improve `Accept` header handling for format negotiation so OpenMetrics is selected correctly by considering whitespace and `q` weights. ([#7208](open-telemetry/opentelemetry-dotnet#7208)) * Emit OpenMetrics exemplars for counters and histogram buckets. ([#7222](open-telemetry/opentelemetry-dotnet#7222)) * Fix incorrect handling of untyped metrics when using OpenMetrics format. ([#7219](open-telemetry/opentelemetry-dotnet#7219)) * Fix Prometheus/OpenMetrics serialization to emit metric and label names containing `_` instead of dropping them and prefixing leading digits. Invalid characters are replaced with `_` instead of being dropped. ([#7209](open-telemetry/opentelemetry-dotnet#7209)) * Add `escaping=underscores` to the `Accept` header handling for content negotiation so OpenMetrics are handled correctly. ([#7209](open-telemetry/opentelemetry-dotnet#7209)) * Omit histogram `_sum` and `_count` in OpenMetrics when negative bucket thresholds are present. ([#7221](open-telemetry/opentelemetry-dotnet#7221)) ... (truncated) Commits viewable in [compare view](open-telemetry/opentelemetry-dotnet@core-1.15.3...core-1.16.0). </details> [![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=OpenTelemetry.Exporter.OpenTelemetryProtocol&package-manager=nuget&previous-version=1.15.3&new-version=1.16.0)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores) Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`. [//]: # (dependabot-automerge-start) [//]: # (dependabot-automerge-end) --- <details> <summary>Dependabot commands and options</summary> <br /> You can trigger Dependabot actions by commenting on this PR: - `@dependabot rebase` will rebase this PR - `@dependabot recreate` will recreate this PR, overwriting any edits that have been made to it - `@dependabot show <dependency name> ignore conditions` will show all of the ignore conditions of the specified dependency - `@dependabot ignore <dependency name> major version` will close this group update PR and stop Dependabot creating any more for the specific dependency's major version (unless you unignore this specific dependency's major version or upgrade to it yourself) - `@dependabot ignore <dependency name> minor version` will close this group update PR and stop Dependabot creating any more for the specific dependency's minor version (unless you unignore this specific dependency's minor version or upgrade to it yourself) - `@dependabot ignore <dependency name>` will close this group update PR and stop Dependabot creating any more for the specific dependency (unless you unignore this specific dependency or upgrade to it yourself) - `@dependabot unignore <dependency name>` will remove all of the ignore conditions of the specified dependency - `@dependabot unignore <dependency name> <ignore condition>` will remove the ignore condition of the specified dependency and ignore conditions </details> Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com> Co-authored-by: Vitor M <4777793+vgmello@users.noreply.github.com>

[Exporter.Prometheus] Fix Accept header handling

ed09ec4

- Update the handling of the `Accept` header to follow the requirements documented in https://prometheus.io/docs/instrumenting/content_negotiation/. - Removed unused code.

martincostello added this to the Exporter.Prometheus Stable Releases milestone Apr 29, 2026

github-actions Bot added pkg:OpenTelemetry.Exporter.Prometheus.AspNetCore Issues related to OpenTelemetry.Exporter.Prometheus.AspNetCore NuGet package pkg:OpenTelemetry.Exporter.Prometheus.HttpListener Issues related to OpenTelemetry.Exporter.Prometheus.HttpListener NuGet package labels Apr 29, 2026

martincostello commented Apr 29, 2026

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[Exporter.Prometheus] Add version constant

d587cc8

- Add a constant for the OpenMetrics version. - Update CHANGELOGs with PR number.

martincostello marked this pull request as ready for review April 29, 2026 10:28

martincostello requested a review from a team as a code owner April 29, 2026 10:28

Copilot AI review requested due to automatic review settings April 29, 2026 10:28

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martincostello marked this pull request as draft April 29, 2026 10:44

martincostello mentioned this pull request Apr 29, 2026

[Exporter.Prometheus] Update Accept/Content-Type handling to consider escaping #7209

Merged

4 tasks

[Exporter.Prometheus] Address feedback

9639341

- Parse `Accept` case-insensitively. - Check `quality` is between 0 and 1.

martincostello marked this pull request as ready for review April 29, 2026 12:41

Kielek approved these changes Apr 30, 2026

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Merge branch 'main' into prometheus-content-negotiation

749cdab

martincostello enabled auto-merge April 30, 2026 10:07

martincostello added this pull request to the merge queue Apr 30, 2026

Merged via the queue into open-telemetry:main with commit c4fa592 Apr 30, 2026
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martincostello deleted the prometheus-content-negotiation branch April 30, 2026 10:40

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[Exporter.Prometheus] Fix Accept header handling#7208

[Exporter.Prometheus] Fix Accept header handling#7208
martincostello merged 4 commits into
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