Allow LogProcessor to mutate log records

[jack-berg]

Resolves #2647.

The simplest possible fix would have been to simply change LogProcessor#emit to accept LogRecord instead of LogData, but I took the opportunity to add some additional detail to make logs more consistent with spans where possible.

Changes include:

• Add TOC
• Rename LogProcessor#emit to LogProcessor#onEmit. This is more consistent with spans (SpanProcessor#onStart, SpanProcessor#onEnd), and avoids confusion by being different than LogEmitter#emit.
• Define LogRecord interface, accepted by LogEmitter#emit
• Define Read/write LogRecord interface, accepted by LogProcessor#onEmit
• Define Readable LogRecord interface, accepted in batches by LogExporter#export
• Add LogExporter#forceFlush method (stubbed out for now) for consistency with SpanExporter#forceFlush

[tigrannajaryan]

LGTM, just one non-blocking question on sync/async behavior.

[jack-berg]

@open-telemetry/specs-logs-approvers please take a look!

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[jack-berg]

Pinging @open-telemetry/specs-logs-approvers. Please take a look! Otel java has already implemented and published this change here.