[Exporter.Geneva] benchmarks nullable

[Kielek]

Towards #894

Changes

[Exporter.Geneva] benchmarks nullable
With fully nullable repository, it will be easier to accomplish #2251. Unneeded #nullable enable are treated as warnings -> errors in .NET9.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.34%. Comparing base (71655ce) to head (6f195f9).
Report is 552 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2254       +/-   ##
===========================================
- Coverage   73.91%   62.34%   -11.58%     
===========================================
  Files         267       45      -222     
  Lines        9615     3986     -5629     
===========================================
- Hits         7107     2485     -4622     
+ Misses       2508     1501     -1007     

Flag	Coverage Δ
unittests-Exporter.Geneva	62.34% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...Metrics/OtlpProtobuf/OtlpProtobufMetricExporter.cs	82.60% <100.00%> (ø)

... and 248 files with indirect coverage changes

[CodeBlanch]

LGTM