feat(otlp): expose export counters via logger params

[LimiNode]

Summary

• Add generic diagnostic logger params:
  • LoggerParam::DroppedLogCount
  • LoggerParam::FailedExportCount
• Expose OtlpHttpLogger dropped-record and failed-export counters through get_string_param(), get_int_param(), and get_float_param().
• Use saturating conversion when exposing unsigned counters through the existing int64_t param API.
• Document OTLP diagnostics usage.

Notes

• This PR is intentionally small and only extends the generic parameter API for existing OTLP counters.
• No direct logging is added inside OTLP error paths, so recursive logging loops remain avoided.

Testing

Not run in this environment.