OpenTelemetry docs should talk only about instrumenting, and not about sdk or exporters.

[cijothomas]

OpenTelemetry crate has dev-dependencies on the SDK/stdout crates as they are demonstrated in the docs.

Proposing to remove that dependency, and modify the doc to only show the API usage, without showing anything related to SDK/Exporters. We should say something like "For the telemetry to be exported, need to install additional crates and point to them", so users are aware that OpenTelemetry crate alone is just the facade which does nothing in the absence of further setup.

This approach is used in https://docs.rs/tracing/latest/tracing/ docs already - it just talks about the tracing facade there, and lists the tracing-subscriber/other crates that are required for the traces to land somewhere.

Similar change is done in #1249. There could be more places, but want to get general feedback on this proposal before further cleanups.

[shaun-cox]

Yep, I'm favor of this approach too.