leverage native async trait in MetricsClient

[gruebel]

Relates #2517

Changes

• I think this is the last asyn_trait we can remove. there is still some usage around HttpClient trait, but this one is used quite often with dynamic dispatch and this can't be handled by the native async capabilities.

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 23 lines in your changes missing coverage. Please review.

Project coverage is 79.1%. Comparing base (06ca4a1) to head (5cf2164).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/metric.rs	0.0%	21 Missing ⚠️
opentelemetry-otlp/src/exporter/http/mod.rs	0.0%	1 Missing ⚠️
opentelemetry-otlp/src/exporter/tonic/mod.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2672     +/-   ##
=======================================
- Coverage   79.2%   79.1%   -0.1%     
=======================================
  Files        122     122             
  Lines      22530   22546     +16     
=======================================
  Hits       17853   17853             
- Misses      4677    4693     +16     

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[cijothomas]

Nice! Thank you.