[CODE HEALTH] Fix gcc warnings in release maintainer build

[dbarker]

Fixes #3983

Changes

• Fix warn_unused_result in otlp_file_client.cc
• Fix maybe-uninitialized in circular_buffer.cc
• Update CI to run the gcc maintainer tests in both release and debug

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (34b4bae) to head (33a80af).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3984   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files         230      230           
  Lines        7299     7299           
=======================================
  Hits         6582     6582           
  Misses        717      717           

Files with missing lines	Coverage Δ
sdk/src/metrics/data/circular_buffer.cc	100.00% <100.00%> (ø)

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[marcalff]

LGTM, see minor comment.