[Exporter.Prometheus] Fix Accept header handling

[martincostello]

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)

[martincostello]

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]

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

[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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