[Code health] Include what you use cleanup, part 5

[marcalff]

Contributes to #2054

Changes

Please provide a brief description of the changes here.

• continued the include-what-you-use cleanup
• total number of warnings decreased from 37 in branch main, to 2 in this PR.
• CI is now enforcing a pass/fail check, with some tolerance (<= 10 warnings)

Note that the cleanup is not completed, as some optional parts (OTLP GRPC) are not covered yet by the build in CI.
This will be handled in subsequent PRs.

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

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	1624d48
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/674124b77eb113000835a082

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.86%. Comparing base (6a77bcd) to head (1624d48).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3140   +/-   ##
=======================================
  Coverage   87.86%   87.86%           
=======================================
  Files         195      195           
  Lines        6151     6151           
=======================================
  Hits         5404     5404           
  Misses        747      747           

Files with missing lines	Coverage Δ
api/include/opentelemetry/nostd/variant.h	66.67% <ø> (ø)
...telemetry/exporters/memory/in_memory_metric_data.h	100.00% <ø> (ø)
exporters/memory/src/in_memory_metric_data.cc	85.00% <ø> (ø)
...rs/memory/src/in_memory_metric_exporter_factory.cc	100.00% <ø> (ø)
...xporters/memory/test/in_memory_metric_data_test.cc	100.00% <ø> (ø)
...ters/memory/test/in_memory_metric_exporter_test.cc	100.00% <ø> (ø)
exporters/memory/test/in_memory_span_data_test.cc	100.00% <ø> (ø)
...orters/memory/test/in_memory_span_exporter_test.cc	100.00% <ø> (ø)
exporters/ostream/test/ostream_log_test.cc	96.18% <ø> (ø)
exporters/ostream/test/ostream_metric_test.cc	100.00% <ø> (ø)
... and 2 more

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[marcalff]

@open-telemetry/cpp-contrib-approvers Ready for review.