Emit deprecation warning when passing Context fields to Logger

We currently need to pass Context fields manually to labkit Loggers in order for them to be logged. This is true for all meta (context) fields except correlation_id. This is confusing and inconsistent. We should simply log all Context fields automatically and stop caller from passing these manually through message.

For now we could start by emitting a warning form labkit whenever we detect this and already merge Context.current anyway, which would just be redundant. Then we can fix callers one by one from passing context fields redundantly.