test(metrics): Trace-connected Metrics (Tests)

[Flash0ver]

based on

• feat(metrics): Trace-connected Metrics (Implementation) #4834

Split into a separate PR, hoping to simplify reviews and discussion.
Heavily based on the tests for Sentry Structured Logging.

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.79%. Comparing base (d730432) to head (941bb41).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           feat/trace-connected-metrics    #4839      +/-   ##
================================================================
+ Coverage                         72.58%   73.79%   +1.20%     
================================================================
  Files                               495      495              
  Lines                             17893    17893              
  Branches                           3523     3523              
================================================================
+ Hits                              12988    13204     +216     
+ Misses                             4048     3829     -219     
- Partials                            857      860       +3     

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