Avoid vec allocation during each export for BatchLogProcessor - Part 2

[utpilla]

For batch export, we create an additional vec just to hold the references to LogRecord and InstrumentationScope. This is needed because the constructor for LogBatch only accepts a slice of tuples that are references to LogRecord and InstrumentationScope. BatchLogProcessor already has a vec of owned LogRecord and InstrumentationScope. This PR is an attempt to reuse that vec.

Changes

• Add a new constructor for LogBatch which accepts a shared reference to a slice of boxed tuple of owned LogRecord and InstrumentationScope.
• No changes to the public API

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

All modified and coverable lines are covered by tests ✅

Project coverage is 77.8%. Comparing base (9a8ad95) to head (94c854a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2488   +/-   ##
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  Coverage   77.8%   77.8%           
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  Files        123     123           
  Lines      22867   22887   +20     
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+ Hits       17804   17824   +20     
  Misses      5063    5063           

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[cijothomas]

Okay to special case to let BatchProcessor avoid alloc. Given no public api change, lets merge.