Add OTEL_EXPORTER environment variable

[iNikem]

What are you trying to achieve?

To simplify the configuration of auto-instrumentations via environment variables, especially for first time users, add new variable OTEL_EXPORTER as a way to configure a list of exporters to use, e.g. OTEL_EXPORTER=zipkin,prometheus,otlp_metrics

Additional context.
This issue assumes that #666 is merged and we have environment variables recommendation in spec.

[carlosalberto]

This would be used mostly by auto-instrumentation, right?

[iNikem]

This would be used mostly by auto-instrumentation, right?

I think yes. Although, even more manual instrumentations this may be a convenient way to externalise the configuration/selection of exporters.

[tigrannajaryan]

I think this is a nice-to-have, not a must-have for 1.0 GA. I suggest to remove release:required-for-ga label.

[anuraaga]

To clarify, if the label is removed, a related PR definitely won't be merged? I don't think this, or SERVICE_NAME, are required, but if they're small in scope it could make sense to still hope they get in before GA to improve user experience of the first release.

[tigrannajaryan]

If there is a PR that is ready and approved I don't see any reason not to merge it even if the label is removed.

Once a spec freeze is declared that will mean we no longer want to accept substantial changes that are not labeled as release:required-for-ga, but I think we are not yet ready to declare a freeze.

[carlosalberto]

Changed it to P1 after talking to the Java Instrumentation Maintainers.

[mtwo]

@anuraaga, are you able to create a PR for this?

[trask]

i discussed with @anuraaga, please assign this to me, thx