Avoid vec allocation during each export for BatchLogProcessor

[utpilla]

We don't have to call Vec::split_off() which causes an allocation.

Changes

• Reuse the preallocated vec when calling export_with_timeout_sync method

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 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.8%. Comparing base (f3b5fd3) to head (263abcd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/logs/log_processor.rs	77.7%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2483   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        123     123           
  Lines      22863   22867    +4     
=====================================
+ Hits       17800   17804    +4     
  Misses      5063    5063           

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