Support more env vars for exporter configuration

[brennan-airtime]

It would be nice to configure things like OTEL_EXPORTER_OTLP_HEADERS, OTEL_EXPORTER_OTLP_CERTIFICATE and OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE like in the spec.

https://opentelemetry.io/docs/concepts/sdk-configuration/general-sdk-configuration/
https://opentelemetry.io/docs/concepts/sdk-configuration/otlp-exporter-configuration/
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md

[djc]

Makes sense, would you be able to contribute a PR?

[brennan-airtime]

For sure. Just wanted to reach out first to make sure I'm not barking up the wrong tree or I didn't miss something obvious.

[cijothomas]

@brennan-airtime Just checking if you are planning to work on this?

[brennan-airtime]

I do still want to do it, but time is a limiting factor...