test: stabilize cancellation metric assertion

[xavierjohn]

Summary

• wait for the actual MeterListener callback after HttpContext.Abort()
• bound the wait to five seconds so a missing metric fails deterministically
• leave production middleware behavior unchanged

Root cause

The TestServer client can observe the aborted response before the server-side middleware disposes MeasuredOperation and records the histogram. The assertion therefore raced the callback even though the measurement was emitted moments later.

Validation

• cancellation metric test passed 30/30 fresh-process runs
• full ASP.NET SLI test suite passed (36/36)

[github-actions]

Test Results

102 tests  ±0   102 ✅ ±0   14s ⏱️ ±0s
  3 suites ±0     0 💤 ±0 
  3 files   ±0     0 ❌ ±0 

Results for commit 5f6d589. ± Comparison against base commit 043ba84.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.17%. Comparing base (043ba84) to head (5f6d589).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #75   +/-   ##
=======================================
  Coverage   83.17%   83.17%           
=======================================
  Files          23       23           
  Lines         434      434           
  Branches       70       70           
=======================================
  Hits          361      361           
  Misses         56       56           
  Partials       17       17           

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