[BUILD] Build break using protoc 3.14

[marcalff]

Fixes #3209

Changes

Please provide a brief description of the changes here.

• Fixed a build break in unit tests, when using protoc 3.14

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.

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[marcalff]

This PR is ready for review.

Build breaks in CI will resolve themselves once #3207 is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.83%. Comparing base (b741815) to head (0159170).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3211   +/-   ##
=======================================
  Coverage   87.83%   87.83%           
=======================================
  Files         195      195           
  Lines        6154     6154           
=======================================
  Hits         5405     5405           
  Misses        749      749