test: Assert traces_sampler called once

[szokeasaurusrex]

Currently, this test only asserts that the traces_sampler is called at least once with the given arguments. However, we would expect it to be called exactly once.

This PR changes the assertion to assert that the traces_sampler was called exactly one time, and that that call included the given arguments.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.79%. Comparing base (cfbb8db) to head (bbc7605).
Report is 2 commits behind head on potel-base.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff               @@
##           potel-base    #4450      +/-   ##
==============================================
- Coverage       84.80%   84.79%   -0.02%     
==============================================
  Files             144      144              
  Lines           14725    14728       +3     
  Branches         2343     2343              
==============================================
  Hits            12488    12488              
  Misses           1523     1523              
- Partials          714      717       +3     

see 4 files with indirect coverage changes