Update serialization format for LogRecord fields.

[lalitb]

Changes

• timestamp (u64) should be converted to a string as per proto-json mapping
• trace_id and span_id (sequence of bytes) should be converted to hex string as per the otlp specs

This is also consistent with SpanData encoding.

This would be eventually tested through #1759.

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 71.6%. Comparing base (348ec9e) to head (1e46f50).
Report is 37 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1771     +/-   ##
=======================================
+ Coverage   71.0%   71.6%   +0.6%     
=======================================
  Files        135     136      +1     
  Lines      20751   20857    +106     
=======================================
+ Hits       14746   14951    +205     
+ Misses      6005    5906     -99     

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