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

[martincostello]

Builds on top of #7208.

Changes

• Add missing SHOULD requirement to specify the escaping value for 1.0.0 protocols.

  For PrometheusText1.0.0 and OpenMetricsText1.0.0 protocols, the Accept header SHOULD include an escaping scheme parameter: escaping=<scheme>

• Update PrometheusSerializer to be compliant with escaping=underscores when using OpenMetrics.

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.87%. Comparing base (0189770) to head (0115430).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7209      +/-   ##
==========================================
+ Coverage   89.86%   89.87%   +0.01%     
==========================================
  Files         272      272              
  Lines       13489    13533      +44     
==========================================
+ Hits        12122    12163      +41     
- Misses       1367     1370       +3     

Flag	Coverage Δ
unittests-Project-Experimental	89.73% <100.00%> (-0.09%)	⬇️
unittests-Project-Stable	89.63% <100.00%> (-0.15%)	⬇️
unittests-UnstableCoreLibraries-Experimental	45.29% <100.00%> (+0.26%)	⬆️

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	98.36% <100.00%> (+0.11%)	⬆️
...tpListener/Internal/PrometheusCollectionManager.cs	87.58% <100.00%> (ø)
...s.HttpListener/Internal/PrometheusHeadersParser.cs	98.33% <100.00%> (+0.18%)	⬆️
...ometheus.HttpListener/Internal/PrometheusMetric.cs	99.43% <100.00%> (+0.06%)	⬆️
...heus.HttpListener/Internal/PrometheusSerializer.cs	98.73% <100.00%> (+0.08%)	⬆️
...s.HttpListener/Internal/PrometheusSerializerExt.cs	98.82% <100.00%> (ø)
....Prometheus.HttpListener/PrometheusHttpListener.cs	88.11% <100.00%> (ø)

... and 3 files with indirect coverage changes

[Copilot]

Pull request overview

Updates the Prometheus exporters’ content negotiation and serialization to align with Prometheus/OpenMetrics “escaping” requirements, particularly escaping=underscores for OpenMetrics 1.0.0.

Changes:

• Emit escaping=underscores in OpenMetrics Content-Type responses and require escaping=underscores when an escaping parameter is explicitly provided in Accept.
• Update serializer label/metric name handling to better match underscore-based escaping rules (including collapsing runs of invalid characters).
• Expand unit + fuzz + integration test coverage for the new behavior and update changelogs.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs	Adds coverage for label-key normalization differences between Prometheus vs OpenMetrics paths.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs	Adds OpenMetrics metric-name escaping/unit normalization test cases (leading digits, collapsing, unsupported chars).
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs	Updates expected OpenMetrics Content-Type to include escaping=underscores.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHeadersParserTests.cs	Adds Accept header parsing test cases for escaping=underscores and rejects unsupported schemes.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.FuzzTests/PrometheusSerializerFuzzTests.cs	Splits fuzz coverage for Prometheus vs OpenMetrics label-key writers and updates reference implementations.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs	Updates Accept negotiation and Content-Type expectations for escaping parameter handling.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListener.cs	Emits OpenMetrics Content-Type with escaping=underscores.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs	Threads openMetricsRequested through tag writing so label-key escaping matches negotiated format.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs	Adds OpenMetrics-aware label-key writing and passes openMetricsRequested through label/tag/scope/target-info writers.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs	Introduces EscapeOpenMetricsName and adjusts OpenMetrics naming/unit suffix logic for underscore escaping.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusHeadersParser.cs	Extends HttpListener-side Accept parsing to validate escaping=underscores when present.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs	Updates internal calls to serializer APIs that now require openMetricsRequested.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents escaping/serialization behavior changes.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterMiddleware.cs	Emits OpenMetrics Content-Type with escaping=underscores and validates escaping parameter in Accept.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Documents escaping/serialization behavior changes.

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[martincostello]

Need to patch the fuzz tests.