[Exporter.Prometheus] Use dictionary to key views

[martincostello]

Changes

• Instead of maintaining separate fields for reviews and timestamps for scrape responses, key on the PrometheusProtocol to support multiple formats and simplify passing options that affect the serializer around in the future (e.g. for [Exporter.Prometheus] Support translation strategy configuration #7156 and [Exporter.Prometheus] Interaction with Translation Strategy #7159).
• Consolidate the buffer and view into a single state object, rather than storing across four dictionaries.
• Rename version properties for consistency to V{number}.

Changes are about ~2% regression for the scrape endpoint, but that's still massively improved compared to before #7279.

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 88.46154% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.89%. Comparing base (f159dfe) to head (1ac912f).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ner/Internal/Shared/PrometheusCollectionManager.cs	87.98%	25 Missing ⚠️
...HttpListener/Internal/Shared/PrometheusProtocol.cs	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7371      +/-   ##
==========================================
- Coverage   90.38%   89.89%   -0.50%     
==========================================
  Files         284      276       -8     
  Lines       15429    14521     -908     
==========================================
- Hits        13946    13054     -892     
+ Misses       1483     1467      -16     

Flag	Coverage Δ
unittests-Project-Experimental	89.80% <87.17%> (-0.12%)	⬇️
unittests-Project-Stable	89.79% <87.17%> (-0.09%)	⬇️
unittests-Solution	89.86% <88.46%> (-0.08%)	⬇️
unittests-UnstableCoreLibraries-Experimental	49.46% <87.17%> (+0.43%)	⬆️

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	100.00% <100.00%> (ø)
...s.HttpListener/Internal/PrometheusHeadersParser.cs	98.85% <100.00%> (ø)
...HttpListener/Internal/Shared/PrometheusExporter.cs	95.23% <ø> (-0.22%)	⬇️
....Prometheus.HttpListener/PrometheusHttpListener.cs	88.67% <100.00%> (ø)
...HttpListener/Internal/Shared/PrometheusProtocol.cs	96.00% <88.88%> (-4.00%)	⬇️
...ner/Internal/Shared/PrometheusCollectionManager.cs	90.80% <87.98%> (-2.59%)	⬇️

... and 10 files with indirect coverage changes

[Copilot]

Pull request overview

This PR refactors the Prometheus exporter scrape response caching/buffering so state is keyed by a richer PrometheusProtocol (media type, escaping, version, etc.), enabling concurrent support for multiple output formats and simplifying future option propagation.

Changes:

• Refactors PrometheusCollectionManager to maintain per-protocol state in a dictionary keyed by PrometheusProtocol, returning a single View per negotiated protocol.
• Extends PrometheusProtocol with IEquatable, GetHashCode, ToString, and adds debug-time validation to constrain key cardinality.
• Updates HttpListener + ASP.NET Core endpoints and test suites to use the protocol-based collection APIs; adds new protocol-focused tests and links them into the ASP.NET Core test project.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusProtocolTests.cs	Adds unit tests for PrometheusProtocol equality/hash behavior and dictionary-key usage.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusCollectionManagerTests.cs	Updates tests to pass/track PrometheusProtocol and adds coverage for cross-protocol shared collection behavior.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests.csproj	Links the new protocol test file into the ASP.NET Core test project.
src/Shared/Shims/Lock.cs	Simplifies the Lock shim declaration used for pre-.NET9 locking compatibility.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListener.cs	Switches scrape path to call EnterCollect(protocol) and write collectionResponse.View.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusProtocol.cs	Adds IEquatable, hashing, debug validation, and renames version constants to V{n}.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusExporter.cs	Removes OpenMetricsRequested mutable state in favor of protocol-driven collection.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/PrometheusCollectionManager.cs	Major refactor: protocol-keyed per-format state, shared active collection across protocols, and new response/result plumbing.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusHeadersParser.cs	Updates version constant names and adds protocol.Validate() after negotiation.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterMiddleware.cs	Switches scrape path to EnterCollect(protocol) and validates protocol in parsing.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/OpenTelemetry.Exporter.Prometheus.AspNetCore.csproj	Links Lock shim into the ASP.NET Core project to support shared code using Lock.

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