Fix protobuf compilation warning in gRPC example

[ThomsonTan]

Fixes #829

Changes

Port and apply the compilation options patch for opentelemetry-cpp to gRPC examples as well.

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

Merging #830 (c8922e9) into main (12946c8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #830   +/-   ##
=======================================
  Coverage   95.50%   95.50%           
=======================================
  Files         156      156           
  Lines        6619     6619           
=======================================
  Hits         6321     6321           
  Misses        298      298           

[lalitb]

thanks for the fix.