Support custom gRPC headers

[slashmo]

Passing custom headers alongside the export gRPC calls is necessary to support exporting directly to a supported backend using e.g. an API key.

We should support the standard OTEL_EXPORTER_OTLP_HEADERS environment variable, which is a comma-separated list of key value pairs, e.g. foo=bar,foobar=baz, as well as passing headers directly to the exporter configuration.

See Also:

• OTLP Exporter: Specifying headers