Support OTEL_EXPORTER environment variable

[jtescher]

See the spec change for details.

[TommyCpp]

I think it's gonna be a tricky one. The PR doesn't include support for external/vendor exporters but all our exporter feels like external exporters since they don't live in the main create.

May need to use Marcos to generate code to configure exporter based on what's available in dependencies.

[jtescher]

@TommyCpp yeah good point. Maybe we hold off on this one until we see if any other language implements this. Could look at what is available, or have another crate that depends on all the standard exporters which adds this functionality, none of these seem like great options currently though.

[TommyCpp]

open-telemetry/opentelemetry.io#4348 seems to conclude that this should be a optional env. If that's the case we could just don't implement it.

[jtescher]

@TommyCpp sounds good, closing for now. If folks want this we can re-open later.