[TEST] Code cleanup in exporter tests

[marcalff]

Fixes # (issue)

Issue

Exporter unit tests contain:

#ifndef OPENTELEMETRY_STL_VERSION

This:

• may disable the tests in CI, depending of the setting for STL_VERSION
• may disable include-what-you-use, likewise
• may disable clang-tidy, likewise

Changes

Please provide a brief description of the changes here.

• Removed the test on OPENTELEMETRY_STL_VERSION.
  • This was an old work around for an issue in GMock. The work around is no longer necessary.
• Applied clang-format to the code
• Resolved all issues reported by include-what-you-use

This ensures that these unit tests are always built and executed.

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 83.25%. Comparing base (a0d2d75) to head (90e8f26).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4158   +/-   ##
=======================================
  Coverage   83.25%   83.25%           
=======================================
  Files         521      521           
  Lines       20379    20379           
=======================================
  Hits        16964    16964           
  Misses       3415     3415           

Files with missing lines	Coverage Δ
exporters/otlp/src/otlp_grpc_exporter_factory.cc	50.00% <ø> (ø)

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