test: Tweak PerformanceMetrics tests for better CI stability

[tippmar-nr]

Minor update to help GCSamplerV2 tests run more reliably in CI.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.79%. Comparing base (25c59e7) to head (8a20465).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3518      +/-   ##
==========================================
- Coverage   81.82%   81.79%   -0.04%     
==========================================
  Files         508      508              
  Lines       34228    34228              
  Branches     4040     4040              
==========================================
- Hits        28007    27996      -11     
- Misses       5254     5266      +12     
+ Partials      967      966       -1     

Flag	Coverage Δ
Agent	82.78% <ø> (-0.04%)	⬇️
Profiler	71.75% <ø> (ø)

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see 3 files with indirect coverage changes

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