Non-blocking batch span processor?

Is your feature request related to a problem? Please describe.

I'd like to increase export throughput, and the most obvious way (at least in my case with a remote ingestion service) is to not block waiting for a response from the ingestion service.

However the spec explicitly disallows calling the exporter concurrently:

Export() will never be called concurrently for the same exporter instance. Export() can be called again only after the current call returns.

So exporters need to implement the non-blocking behavior themselves.

But the only way to tell the batch span processor not to block, is to return an immediately completed CompletableResultCode, instead of a real CompletableResultCode.

Which sort of defeats the purpose of the async-friendly CompletableResultCode return type from export.

In any case though, if this is a supported pattern (returning an immediately completed CompletableResultCode from the exporter), it would be nice for the batch span processor to flush the span exporter after if flushes itself, or to document on the batch span processor's flush() method that you may still need to flush the underlying span exporter after calling that method.

Start with the batch span processor's flush() method and the exporter Export() contract linked in the issue. Determine whether non-blocking behavior should be supported by processor flushing or documented; done would be an agreed implementation or documentation change.