Geneva Exporter: Improve multithreaded support

[mattsains]

Changes

Reviewed the trace and log exporter code and guarded against some potential race conditions.

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 71.56%. Comparing base (c79d8a6) to head (eb205a1).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3881      +/-   ##
==========================================
+ Coverage   71.43%   71.56%   +0.13%     
==========================================
  Files         457      447      -10     
  Lines       17879    17827      -52     
==========================================
- Hits        12771    12758      -13     
+ Misses       5108     5069      -39     

Flag	Coverage Δ
unittests-Exporter.Geneva	54.45% <100.00%> (-0.01%)	⬇️
unittests-Instrumentation.Cassandra	?

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Files with missing lines	Coverage Δ
...er.Geneva/Internal/MsgPack/MsgPackTraceExporter.cs	94.19% <100.00%> (-0.02%)	⬇️

... and 10 files with indirect coverage changes

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