[Exporter.Prometheus] Use Stopwatch for scrape cache freshness check

[unsafePtr]

Changes

Follow-up to #7193. The scrape-response cache added in #7189 still uses DateTime.UtcNow for its freshness check, which can keep stale entries indefinitely on a backward NTP step or evict valid ones on a forward step. Switch to Stopwatch.GetElapsedTime, matching the pattern #7193 applied elsewhere and mirroring dotnet/runtime#127303 which made the same change for EventCounter's polling timer.
CollectionResponse.GeneratedAtUtc is unchanged.

StopwatchExtensions.cs (added in #7193) is linked into the two Prometheus packages.

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)

[martincostello]

I'm getting HTTP 500s from GitHub trying to post review comments, so I'm trying a PR comment instead:

I think the CHANGELOG entries are overly-verbose for a user-facing note. Can you slim them down please?

Also they need a link to this PR like all the other entries link to their corresponding PR.

[unsafePtr]

@martincostello, thank you. Updated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.91%. Comparing base (58b0202) to head (d82ce59).
⚠️ Report is 15 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7253      +/-   ##
==========================================
+ Coverage   89.88%   89.91%   +0.03%     
==========================================
  Files         273      273              
  Lines       13755    13787      +32     
==========================================
+ Hits        12363    12396      +33     
+ Misses       1392     1391       -1     

Flag	Coverage Δ
unittests-Project-Experimental	89.78% <100.00%> (-0.03%)	⬇️
unittests-Project-Stable	89.80% <100.00%> (+0.02%)	⬆️
unittests-UnstableCoreLibraries-Experimental	45.81% <100.00%> (+0.07%)	⬆️

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

Files with missing lines	Coverage Δ
...tpListener/Internal/PrometheusCollectionManager.cs	91.56% <100.00%> (+0.33%)	⬆️

... and 2 files with indirect coverage changes

[github-actions]

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