New spec proposal

[trask]

@jkwatson this is my understanding of the kind of change we'd make if open-telemetry/opentelemetry-specification#2452 goes through

[anuraaga]

We definitely! need export callers to have a way to know if the export succeeded or not.

I don't see any new language in that spec PR that would need changes in the Java SDK - we already don't block when Export is called. As we know, the concurrency issue is in the span processor, not exporter.