[Metrics] Remove unnecessary box

[utpilla]

Changes

• Remove unnecessary box

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 79.5%. Comparing base (c73c938) to head (ae29664).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/manual_reader.rs	0.0%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2376   +/-   ##
=====================================
  Coverage   79.5%   79.5%           
=====================================
  Files        123     123           
  Lines      21479   21479           
=====================================
  Hits       17084   17084           
  Misses      4395    4395           

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