[CMAKE] Add pkg-config support for OpenTelemetry proto installation

[owent]

Fixes #4182

Changes

• [CMAKE] Add pkg-config support for OpenTelemetry proto installation

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

[Copilot]

Pull request overview

Adds pkg-config metadata for the installed OpenTelemetry protobuf/gRPC protobuf libraries so non-CMake consumers can obtain the required -DOPENTELEMETRY_PROTO_API=... compile definition via pkg-config --cflags, addressing the undefined macro issue reported in #4182.

Changes:

• Add an include guard to the shared cmake/pkgconfig.cmake helper to prevent repeated inclusion.
• Generate and install opentelemetry_proto.pc (and opentelemetry_proto_grpc.pc when WITH_OTLP_GRPC=ON) during OPENTELEMETRY_INSTALL.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmake/pkgconfig.cmake	Adds include_guard(GLOBAL) to avoid re-including the pkg-config helper module.
cmake/opentelemetry-proto.cmake	Installs generated proto headers and installs new pkg-config .pc files for opentelemetry_proto (+ opentelemetry_proto_grpc when enabled).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.90%. Comparing base (a56f0e2) to head (ca648f9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4192      +/-   ##
==========================================
+ Coverage   82.89%   82.90%   +0.02%     
==========================================
  Files         405      405              
  Lines       17292    17292              
==========================================
+ Hits        14332    14334       +2     
+ Misses       2960     2958       -2     

see 1 file with indirect coverage changes

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