[Prometheus.HttpListener] Make loop async

[martincostello]

Changes

Make the worker task for processing requests asynchronous.

Mainly this is just me trying to get rid of the .Result.

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 89.08%. Comparing base (7dd46f4) to head (072bd10).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7121      +/-   ##
==========================================
- Coverage   89.09%   89.08%   -0.01%     
==========================================
  Files         271      271              
  Lines       13065    13074       +9     
==========================================
+ Hits        11640    11647       +7     
- Misses       1425     1427       +2     

Flag	Coverage Δ
unittests-Project-Experimental	88.89% <100.00%> (-0.18%)	⬇️
unittests-Project-Stable	89.08% <100.00%> (+0.06%)	⬆️
unittests-UnstableCoreLibraries-Experimental	40.89% <100.00%> (+0.13%)	⬆️

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

Files with missing lines	Coverage Δ
....Prometheus.HttpListener/PrometheusHttpListener.cs	92.15% <100.00%> (+7.21%)	⬆️

... and 2 files with indirect coverage changes

[Copilot]

Pull request overview

This PR updates the Prometheus HttpListener worker loop to be asynchronous (removing blocking .Result/Wait patterns where possible) and adds/updates tests to validate stop behavior and concurrent scrape handling.

Changes:

• Convert the listener worker loop to ProcessingLoopAsync and use async waiting for GetContextAsync on modern TFMs.
• Add new tests for stop behavior with an in-flight scrape and for concurrent scrapes.
• Simplify a parallel test path by using WaitAsync for completion under NET.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListener.cs	Replaces the synchronous worker proc with an async processing loop and async context acquisition.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs	Adds tests for stop behavior and concurrent scrapes; introduces a helper to start the listener on a random port.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusCollectionManagerTests.cs	Uses WaitAsync for the Parallel.ForAsync completion path under NET.

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