[Spec] Tracers should reference an InstrumentationLibrary rather than a Resource

[dyladan]

Spec PR merged: open-telemetry/opentelemetry-specification#616

@mwear since you wrote the spec do you want to take this? If so, please assign yourself. If there is nothing to do, feel free to close this ticket.

[mwear]

It's somewhat in limbo based on: open-telemetry/opentelemetry-proto#150. I think we can move forward with these changes, and make updates if and when the protos are changed.