docs(metric): document public interfaces with versioning policy

[wisdommatt]

• Add documentation comments to exported metric interfaces stating they cannot be changed.

Resolves #3384

[linux-foundation-easycla]

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• ✅ login: wisdommatt / name: Wisdom Matthew (54bcf69)

[Aneurysm9]

I'm good with this, with the caveat that #3277 still needs to be addressed and we'll need to apply the resolution to these interfaces as well.

[codecov]

Codecov Report

Merging #3441 (2e00439) into main (6dccc07) will decrease coverage by 0.0%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3441     +/-   ##
=======================================
- Coverage   77.8%   77.8%   -0.1%     
=======================================
  Files        164     164             
  Lines      11494   11494             
=======================================
- Hits        8948    8946      -2     
- Misses      2343    2345      +2     
  Partials     203     203             

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	81.1% <0.0%> (-0.9%)	⬇️

[dmathieu]

If we document this for metric interfaces, should we also document it for trace ones ?

[MrAlias]

If we document this for metric interfaces, should we also document it for trace ones ?

It was done in #2012