Clarify the SDK environment variables requirement

[carlosalberto]

We need to clarify the expectations for the SDK environment variables, as some of them are meant to be supported, GA-wise, only for auto-instrumentation (although the plan is to support them in the SDKs directly after GA). Examples of them are OTEL_TRACE_SAMPLER and OTEL_PROPAGATORS.

@cijothomas left this comment in #1190 which can be used as inspiration 😉

SDKs MAY choose to allow configuration via the environment variables in this specification, but are not required to. If they do, they SHOULD use the names listed in this document.

[anuraaga]

I've been wondering about this in the other direction - why have env variables only for auto instrumentation? SDK may as well support them for non-auto users I guess. In Java I've been wanting to move a lot of the initialization logic to SDK since it seems like a small change with big benefit to SDK users.

[Oberon00]

In Java I've been wanting to move a lot of the initialization logic to SDK since it seems like a small change with big benefit to SDK users.

It may also be a security issue if your app suddenly starts exporting telemetry because some environment variables are set, especially if it did not do so before.

[Oberon00]

I would thus prefer:

SDKs MUST NOT use any environment information automatically. SDKs MAY implement a way to request auto-configuration based on environment variables. If they do, they SHOULD use the names listed in this document.