[Metrics API] Mark KeyValue types non-exhaustive

[utpilla]

Changes

• Mark KeyValue related structs and enums as non_exhaustive to allow us to add newer fields or variants if needed in future
• Here the list of structs and enums that were marked non_exahustive:
  • struct KeyValue
  • struct Key
  • struct StringValue
  • enum Value
  • enum Array

Merge requirement checklist

• CONTRIBUTING guidelines followed
• 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 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.1%. Comparing base (a47b429) to head (0acdc2f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-proto/src/transform/common.rs	0.0%	2 Missing ⚠️
opentelemetry-stdout/src/common.rs	0.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2228     +/-   ##
=======================================
- Coverage   79.1%   79.1%   -0.1%     
=======================================
  Files        121     121             
  Lines      21180   21184      +4     
=======================================
- Hits       16774   16772      -2     
- Misses      4406    4412      +6     

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