[CONFIGURATION] File configuration - implement log_level

[marcalff]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

• Implement the missing log_level property, not implemented for C++
• Follow up on: Update .log_level to be enum opentelemetry-configuration#447

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 89.96%. Comparing base (7d41406) to head (5dd56ec).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3810   +/-   ##
=======================================
  Coverage   89.96%   89.96%           
=======================================
  Files         225      225           
  Lines        7167     7167           
=======================================
  Hits         6447     6447           
  Misses        720      720           

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[marcalff]

This resolves:

• https://github.com/open-telemetry/opentelemetry-configuration/blob/e4d2fdb07539ada5d1d5ab929f71a45705792fa3/schema/meta_schema_language_cpp.yaml#L159

[marcalff]

Added reminders for known missing items from the spec.

See https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema/meta_schema_language_cpp.yaml