refactor: avoid instrumentation_scope clone for every single span processor

[taisho6339]

Fixes #1125

Changes

Optimize InstrumentationScope handling by wrapping it in Arc instead of cloning for each span processor.

Previously, when a span ended, the InstrumentationScope (approximately 120 bytes) was cloned once for each SpanProcessor, causing unnecessary allocations and memory copies. This change wraps InstrumentationScope in Arc at tracer creation time, so subsequent clones are just atomic reference count increments.

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

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.8%. Comparing base (df412fe) to head (e2ecd0c).

Files with missing lines	Patch %	Lines
opentelemetry-proto/src/transform/trace.rs	85.7%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3267   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        129     129           
  Lines      23203   23208    +5     
=====================================
+ Hits       18750   18755    +5     
  Misses      4453    4453           

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