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.

Please upload report for BASE (main@b8380eb). Learn more about missing BASE report.

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        ?   79.4%           
======================================
  Files           ?     122           
  Lines           ?   21708           
  Branches        ?       0           
======================================
  Hits            ?   17255           
  Misses          ?    4453           
  Partials        ?       0           

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[cijothomas]

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

[lalitb]

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

updated changelog.