add Logger.emitEvent

[brettmc]

spec 1.40.0 deprecates the event api+sdk in favour of using the log API: open-telemetry/opentelemetry-specification#4319

Depends on proto 1.5.0 (for event_name top-level field)

edit: on-hold, EmitEvent has been removed from spec and no replacement yet

[brettmc]

Closing, there is no replacement. Now, the log API is user-facing.