[Metrics API] Mark structs non-exhaustive

[utpilla]

Changes

• Mark Meter and all the instruments structs as non_exhaustive

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

All modified and coverable lines are covered by tests ✅

Project coverage is 79.0%. Comparing base (caa4246) to head (d75fd37).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2202     +/-   ##
=======================================
- Coverage   79.1%   79.0%   -0.1%     
=======================================
  Files        121     121             
  Lines      21009   21009             
=======================================
- Hits       16620   16616      -4     
- Misses      4389    4393      +4     

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