[BUILD] Remove gRPC header including in OtlpGrpcClientFactory.

[owent]

Fixes #3320

Changes

• Remove the gRPC header dependency of OtlpGrpcClientFactory

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.52%. Comparing base (63220ad) to head (6b3b613).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3321      +/-   ##
==========================================
+ Coverage   89.51%   89.52%   +0.02%     
==========================================
  Files         210      210              
  Lines        6526     6526              
==========================================
+ Hits         5841     5842       +1     
+ Misses        685      684       -1     

see 1 file with indirect coverage changes

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[dbarker]

Looks good to me. Can the methods of the grpc client be protected?

[owent]

Looks good to me. Can the methods of the grpc client be protected?

To prevent the inclusion of gRPC headers in the future, I added macro checks to the factory unit test code. Since gRPC client methods must be used in OTLP gRPC exporters, making them protected would require numerous friend classes for exporters and unit tests, which would hinder maintainability.

[marcalff]

The factory returns std::shared_ptr<OtlpGrpcClient>, so I think the user code will still need to see the destructor of OtlpGrpcClient at some point.

The header file for OtlpGrpcClient might need additional cleanup.

In any case, this change in the client factory is needed, so approving this PR.

[dbarker]

A note in otlp_grpc_client.h and related markdown documents will also help users avoid going down the path of including the file in their projects and trying to link grpc directly.