Make LogRecord fields private and add getters for encapsulation

[lalitb]

Changes

The PR refactors the LogRecord struct by making all its fields crate-private for better abstraction. Public getter methods have been added to provide controlled access to the fields.

TBD - Will add changelog, if the changes are agreed.

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 85.24590% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.5%. Comparing base (6b71301) to head (206dd7e).

Files with missing lines	Patch %	Lines
opentelemetry-stdout/src/logs/exporter.rs	0.0%	8 Missing ⚠️
opentelemetry-proto/src/transform/logs.rs	92.3%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2314   +/-   ##
=====================================
  Coverage   79.5%   79.5%           
=====================================
  Files        123     123           
  Lines      21492   21509   +17     
=====================================
+ Hits       17096   17113   +17     
  Misses      4396    4396           

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[cijothomas]

LGTM with changelog as this is breaking export/processor authors.