Add helper function to get serializable LogRecord

[nanorobocop]

Is your feature request related to a problem?
OTLP log format consists of nested objects LogsData -> []ResourceLogs -> []ScopeLogs -> []LogRecord (proto).

There's helper function to obtain serializable ResourceLogs structure - LogsToProto.
But there's no such function to obtain serializable LogRecord.
Serializable LogRecord may require for exporters that want to deal with individual log records, like in open-telemetry/opentelemetry-collector-contrib#12623

Describe the solution you'd like
Add helper function to get serializable LogRecord.

Describe alternatives you've considered
no

Additional context
no

[bogdandrutu]

Per the comments in the initial issue open-telemetry/opentelemetry-collector-contrib#12623 (comment) we are closing this and will not provide this functionality for the moment.