SDK: should Resource initialization use env vars only?

[fbogsany]

#86 (comment):

In the java SDK the resource is final: https://github.com/open-telemetry/opentelemetry-java/blob/master/sdk/src/main/java/io/opentelemetry/sdk/trace/TracerSdk.java#L50 and assigned as an EnvVarResource, which we don't yet have. We could consider doing the same and turn this into a reader.

Should we allow manual configuration of Resource or should it be entirely automatic based on environment variables, like the Java SDK?

[fbogsany]

Bump. Is this still relevant or is this resolved by #263?

[fbogsany]

open-telemetry/oteps#111 will clarify this. At present, we support setting this in a couple different ways, which is probably correct. This may change -- we will revisit it when OTEP 111 is resolved.