[Prometheus.HttpListener] Support scrape HTTP response caching

[martincostello]

Changes

Add support for caching HTTP responses in OpenTelementry.Exporter.Prometheus.HttpListener.

This creates parity with the ASP.NET Core exporter instead of forcing it to be disabled.

If we'd rather keep the current behaviour by default, I can change the default back to 0.

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 89.58%. Comparing base (86f0c3f) to head (dff3178).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7189      +/-   ##
==========================================
+ Coverage   89.25%   89.58%   +0.33%     
==========================================
  Files         271      272       +1     
  Lines       13093    13445     +352     
==========================================
+ Hits        11686    12045     +359     
+ Misses       1407     1400       -7     

Flag	Coverage Δ
unittests-Project-Experimental	89.38% <100.00%> (+0.25%)	⬆️
unittests-Project-Stable	89.38% <100.00%> (+0.24%)	⬆️
unittests-UnstableCoreLibraries-Experimental	40.99% <100.00%> (+0.05%)	⬆️

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

Files with missing lines	Coverage Δ
...theusHttpListenerMeterProviderBuilderExtensions.cs	81.25% <100.00%> (ø)
...heus.HttpListener/PrometheusHttpListenerOptions.cs	100.00% <100.00%> (ø)

... and 12 files with indirect coverage changes

[Copilot]

Pull request overview

This PR adds configurable scrape HTTP response caching for the Prometheus HttpListener exporter, bringing it to parity with the ASP.NET Core exporter and enabling caching by default.

Changes:

• Introduces PrometheusHttpListenerOptions.ScrapeResponseCacheDurationMilliseconds (default 300ms; 0 disables caching).
• Plumbs the configured cache duration into the PrometheusExporter created by AddPrometheusHttpListener.
• Updates/extends unit tests and changelog/public API baselines for the new option.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerMeterProviderBuilderExtensionsTests.cs	Adds a unit test asserting configured cache duration flows into the exporter.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusCollectionManagerTests.cs	Updates theory inputs to align with new default caching behavior for HttpListener.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListenerOptions.cs	Adds new cache duration option and sets default to 300ms.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListenerMeterProviderBuilderExtensions.cs	Passes the new HttpListener option through to the exporter options.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents the new caching support in Unreleased.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/.publicApi/PublicAPI.Unshipped.txt	Adds new public API surface for the cache duration property.

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