[Logs API] Mark Anyvalue enum as non-exhaustive

[lalitb]

Changes

Mark AnyValue enum non-exhaustive (in-line with the changes in #2228)

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 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.1%. Comparing base (ea4b5e4) to head (ebbfda1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-proto/src/transform/logs.rs	0.0%	1 Missing ⚠️
opentelemetry-stdout/src/common.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2230     +/-   ##
=======================================
- Coverage   79.1%   79.1%   -0.1%     
=======================================
  Files        121     121             
  Lines      21082   21084      +2     
=======================================
- Hits       16680   16678      -2     
- Misses      4402    4406      +4     

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