[Metrics API] Mark no-op structs as pub(crate)

[utpilla]

Changes

• This PR reduces the number of public APIs exposed by removing SDK's dependency on using API's no-op versions of Meter and instruments
• The intended behavior is retained by SDK keeping its own version of these no-op structs

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 67.85714% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.0%. Comparing base (f1a6e16) to head (0727bef).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/noop.rs	75.0%	6 Missing ⚠️
opentelemetry/src/metrics/noop.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2203     +/-   ##
=======================================
- Coverage   79.1%   79.0%   -0.1%     
=======================================
  Files        121     122      +1     
  Lines      21009   21033     +24     
=======================================
  Hits       16620   16620             
- Misses      4389    4413     +24     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cijothomas]

LGTM. This is still adhering to the spirit of the spec. Happy to see the trimmed down public API as we move to declare Metrics API as RC/Stable soon.