Are env vars like OTEL_EXPORTER_OTLP_HEADERS supported now (especially for grpc)?

[WieslerAA]

OTEL_EXPORTER_OTLP_HEADERS is referenced in both the tonic files and the http files. There are also comments stating that grpc does not support those flags, however, they are used there, and it seems to be implemented for tonic.

Are the comments just outdated?

This also relates to #984