[API] Deprecate opentelemetry::plugin

[marcalff]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

• Deprecated files in API, in the opentelemetry::plugin namespace
  • Added warnings in the files affected, and an error in no-deprecated builds.
• Updated the DEPRECATED.md documentation
• Commented out plugin tests and examples from CMake and Bazel
• Disabled the plugin specific CI

The code is commented out for now, for easier rollback if so required.
File removal, including tests, examples and CI, to be done in the final removal PR.

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 82.05%. Comparing base (d63ed0a) to head (a01e279).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4021      +/-   ##
==========================================
+ Coverage   81.94%   82.05%   +0.12%     
==========================================
  Files         389      385       -4     
  Lines       15927    15891      -36     
==========================================
- Hits        13049    13038      -11     
+ Misses       2878     2853      -25     

see 5 files with indirect coverage changes

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[ThomsonTan]

@lalitb @esigo, could you please take a look? The plugin interface has been there from the very beginning.

[esigo]

LGTM
Thanks for the PR :)

[lalitb]

Thanks, will have a look now.

[lalitb]

Good to deprecate, but as per the deprecation process, we should be adding this in each public header deprecated:

#ifdef OPENTELEMETRY_NO_DEPRECATED_CODE
#  error "header <opentelemetry/foo.h> is deprecated."
#endif

[marcalff]

Good to deprecate, but as per the deprecation process, we should be adding this in each public header deprecated:

#ifdef OPENTELEMETRY_NO_DEPRECATED_CODE
#  error "header <opentelemetry/foo.h> is deprecated."
#endif

I don't think anyone will see this, it supposes building with OPENTELEMETRY_NO_DEPRECATED_CODE in the first place.

Added warnings in all cases for visibility, as well as the error in no-deprecated builds, for more safety.

Note that I commented out CMake and bazel then, so we don't see this ourselves.

[lalitb]

I don't think anyone will see this, it supposes building with OPENTELEMETRY_NO_DEPRECATED_CODE in the first place.

Oh yes, My earlier comment was based on the current docs/deprecation-process.md guidance for deprecated public headers, which explicitly calls for the OPENTELEMETRY_NO_DEPRECATED_CODE error path. So that part was about following the documented process as written.

That said, I agree that the OPENTELEMETRY_NO_DEPRECATED_CODE check alone is only visible in verification builds, so adding warnings in normal builds as well is a better outcome. Keeping warnings for general visibility and the hard error for no-deprecated builds makes sense to me.

I think we should do the follow-up PR to update docs/deprecation-process.md so the documented header deprecation pattern matches what we now consider the preferred implementation.