docs(otlp-transformer): mark log APIs experimental

[JacksonWeber]

Summary

• mark LogsExporterMetricsHelper as experimental because it exposes ReadableLogRecord[]
• make the existing experimental annotations on JsonLogsSerializer and ProtobufLogsSerializer TypeDoc-compatible
• leave internal APIs and self-contained log export response interfaces unchanged, per the maintainer clarification

Validation

• targeted ESLint on the three changed files
• targeted TypeScript no-emit check using repository compiler options
• TypeDoc conversion confirming exactly the three intended public exports are marked @experimental
• git diff --check

Closes #4584

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.02%. Comparing base (756f49a) to head (45ee692).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7016   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         409      409           
  Lines       14304    14304           
  Branches     3276     3276           
=======================================
  Hits        13593    13593           
  Misses        711      711           

Files with missing lines	Coverage Δ
...al/packages/otlp-transformer/src/logs/json/logs.ts	100.00% <ø> (ø)
...ackages/otlp-transformer/src/logs/protobuf/logs.ts	100.00% <ø> (ø)

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