[Exporter.Prometheus] Add ResourceConstantLabels

[martincostello]

Fixes #7437

Changes

Add support for emitting resource attributes as metric labels based on a user-supplied predicate.

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

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.41%. Comparing base (68c29d8) to head (dda4fd1).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ometheus.AspNetCore/PrometheusAspNetCoreOptions.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7471      +/-   ##
==========================================
+ Coverage   90.18%   90.41%   +0.22%     
==========================================
  Files         287      287              
  Lines       15553    16172     +619     
==========================================
+ Hits        14027    14622     +595     
- Misses       1526     1550      +24     

Flag	Coverage Δ
unittests-Project-Experimental	90.04% <96.77%> (-0.17%)	⬇️
unittests-Project-Stable	90.09% <96.77%> (-0.14%)	⬇️
unittests-UnstableCoreLibraries-Experimental	50.69% <96.77%> (+0.10%)	⬆️

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

Files with missing lines	Coverage Δ
...ner/Internal/Shared/PrometheusCollectionManager.cs	90.73% <100.00%> (+0.35%)	⬆️
...HttpListener/Internal/Shared/PrometheusExporter.cs	100.00% <100.00%> (ø)
...tener/Internal/Shared/PrometheusExporterOptions.cs	100.00% <ø> (ø)
...ernal/Shared/Serialization/TextFormatSerializer.cs	99.65% <100.00%> (+0.16%)	⬆️
...hared/Serialization/TextFormatSerializerOptions.cs	100.00% <100.00%> (ø)
...theusHttpListenerMeterProviderBuilderExtensions.cs	82.85% <100.00%> (+0.50%)	⬆️
...heus.HttpListener/PrometheusHttpListenerOptions.cs	100.00% <ø> (ø)
...ometheus.AspNetCore/PrometheusAspNetCoreOptions.cs	50.00% <50.00%> (ø)

... and 10 files with indirect coverage changes

[Copilot]

Pull request overview

Adds support in the Prometheus exporters (ASP.NET Core scraping endpoint + HttpListener) to emit selected OpenTelemetry Resource attributes as constant metric labels, controlled by a user-supplied predicate, aligning with the resource_constant_labels option described in the Prometheus exporter spec.

Changes:

• Introduces ResourceConstantLabels on both PrometheusAspNetCoreOptions and PrometheusHttpListenerOptions and wires it into exporter serialization.
• Extends the text format serializer pipeline to merge configured resource labels with per-metric tag labels (handling key collisions via existing label-merge logic).
• Adds integration tests + snapshots covering “all / one key / none” resource-label selection.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.RunWithResourceConstantLabelsConfigured_filter=service_name.verified.text	New snapshot validating resource labels added to metrics using a key-filter predicate.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.RunWithResourceConstantLabelsConfigured_filter=none.verified.text	New snapshot validating no resource labels are added when predicate excludes all.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.RunWithResourceConstantLabelsConfigured_filter=all.verified.text	New snapshot validating all resource attributes become constant metric labels.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.RunHttpServerWithResourceConstantLabelsConfigured_filter=service_name.verified.text	New snapshot validating HttpListener server output with filtered resource labels.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.RunHttpServerWithResourceConstantLabelsConfigured_filter=none.verified.text	New snapshot validating HttpListener server output with no resource labels.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.RunHttpServerWithResourceConstantLabelsConfigured_filter=all.verified.text	New snapshot validating HttpListener server output with all resource labels.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs	Updates serializer options construction to include the new resource-labels option parameter.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs	Adds integration test coverage for HttpListener ResourceConstantLabels behavior.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs	Adds integration test coverage for ASP.NET Core ResourceConstantLabels behavior.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/README.md	Documents ResourceConstantLabels for HttpListener exporter configuration.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListenerOptions.cs	Adds public ResourceConstantLabels option (predicate) to HttpListener options.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListenerMeterProviderBuilderExtensions.cs	Wires PrometheusHttpListenerOptions.ResourceConstantLabels into exporter construction.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/Serialization/TextFormatSerializerOptions.cs	Extends serializer options to carry computed resource constant labels into serialization.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/Serialization/TextFormatSerializer.cs	Emits resource constant labels on each metric and ensures correct collision/merge behavior.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusExporterOptions.cs	Adds internal shared option ResourceConstantLabels used by both hosting models.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusExporter.cs	Stores ResourceConstantLabels on the exporter for use during collection/serialization.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusCollectionManager.cs	Computes and caches filtered resource labels and passes them into serialization.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Adds changelog entry for the new PrometheusHttpListenerOptions.ResourceConstantLabels behavior.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/.publicApi/PublicAPI.Unshipped.txt	Tracks new public API surface for HttpListener options property.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/README.md	Documents ResourceConstantLabels for ASP.NET Core exporter configuration.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusAspNetCoreOptions.cs	Adds public ResourceConstantLabels option (predicate) to ASP.NET Core options.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Adds changelog entry for the new PrometheusAspNetCoreOptions.ResourceConstantLabels behavior.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/.publicApi/PublicAPI.Unshipped.txt	Tracks new public API surface for ASP.NET Core options property.

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[Kielek]

LGTM with some direct feedback.