[RFC] Functional Interface pattern for Collector extension APIs

[jmacd]

Description

Replaces #13263

Documents the interface pattern used in core interfaces (e.g., component, consumer, extensions), which enables safe interface evolution as described in this RFC. This documents an existing pattern so that it can be applied more consistently across our code base.

Testing Documentation

This interface pattern has been checked and documented using examples from PR #13241.

[jmacd]

I've referred to my former attempt at this many times in the months since I first opened it, so here it is again. Following feedback from @evan-bradley I changed the name to "Functional Interface" pattern. Any name will do, really.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.68%. Comparing base (f96a135) to head (4b8a625).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13902      +/-   ##
==========================================
+ Coverage   91.66%   91.68%   +0.01%     
==========================================
  Files         652      652              
  Lines       42506    42506              
==========================================
+ Hits        38964    38972       +8     
+ Misses       2734     2728       -6     
+ Partials      808      806       -2     

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[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[jmacd]

open-telemetry/opentelemetry-collector-contrib#42573 is an example of an ad-hoc extension mechanism in collector-contrib: it might be valuable to apply these patterns in that repository.