Support export concurrency

[MrAlias]

Problem Statement

The specification now allows concurrent exports. Including this in this project could provide export throughput for users.

Proposed Solution

We should evaluate and possibly include concurrent algorithms for our exporters:

• otlp
• zipkin

Alternatives

Leave things the way they are.

[MadVikingGod]

This is a weird change because the spec still states:

Export() will never be called concurrently for the same exporter instance.

But that the act of exporting can be done concurrently. So what does that mean for the error returned by ExportSpans()? If the export is synchronous, even if it's concurrent then we have to put the concurrent logic in the SpanProcessor, because our SpanProcessors and flush/shutdown don't continue processing. If the export is now asynchronous then the error can't represent the final status of the export, this will also have impact on shutdown and cleanup logic. E.g. what happens if we call shutdown with outstand Export in flight; how will that impact our examples?

I think we need a different interface to gracefully enable asynchronous Exports, and thus concurrent exports.