[tests] Clean up class structure in metrics tests

[CodeBlanch]

Follow-up to #5956

Changes

• A bunch of class structure was introduced to be able to test everything with and without delta metricpoint reclaim being enabled. Now that it is always enabled, this structure isn't required.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.70%. Comparing base (6250307) to head (3096d21).
Report is 381 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5960      +/-   ##
==========================================
+ Coverage   83.38%   86.70%   +3.32%     
==========================================
  Files         297      263      -34     
  Lines       12531    11931     -600     
==========================================
- Hits        10449    10345     -104     
+ Misses       2082     1586     -496     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.50% <ø> (?)
unittests-Project-Stable	86.63% <ø> (?)

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see 242 files with indirect coverage changes

[xiang17]

The MetricPointReclaimTestsBase and MetricOverflowAttributeTestsBase classes are also the same pattern but missing in this PR.

[CodeBlanch]

The MetricPointReclaimTestsBase and MetricOverflowAttributeTestsBase classes are also the same pattern but missing in this PR.

@xiang17 Updated!