[TEST] Code cleanup in exporter tests

[marcalff]

Fixes # (issue)

Issue

Exporter unit tests contain:

#ifndef OPENTELEMETRY_STL_VERSION

This:

• disable the tests in CI, typically run without a specific STL
• disable include-what-you-use

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

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 82.83%. Comparing base (17c4723) to head (3f21116).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4158      +/-   ##
==========================================
+ Coverage   82.82%   82.83%   +0.02%     
==========================================
  Files         406      406              
  Lines       16913    16913              
==========================================
+ Hits        14007    14009       +2     
+ Misses       2906     2904       -2     

see 1 file with indirect coverage changes

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