fix flaky unit tests

[mariomac]

There is still a small chance that TestTraceSampling fails, as it's just a probabilistic matter. But should greatly reduce the frequency of failure.

Also fixes TestSpanAttributeFilterNode, which wrongly assumed that two consecutive metric reports would belong to different entities.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.86%. Comparing base (77046c4) to head (1817516).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1421      +/-   ##
==========================================
+ Coverage   72.30%   80.86%   +8.56%     
==========================================
  Files         145      146       +1     
  Lines       14893    14925      +32     
==========================================
+ Hits        10768    12069    +1301     
+ Misses       3405     2265    -1140     
+ Partials      720      591     -129     

Flag	Coverage Δ
integration-test	59.02% <ø> (-0.10%)	⬇️
k8s-integration-test	59.86% <ø> (-0.18%)	⬇️
oats-test	33.90% <ø> (-0.08%)	⬇️
unittests	51.57% <ø> (?)

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[grcevski]

LGTM!