prometheus
Clarify internal usage of empty label values by federation
Ref: https://github.com/prometheus/prometheus/issues/11024
Attempt to clarify the existing usage of empty label values by the federation system.
More details of that use case here: https://github.com/prometheus/prometheus/issues/8726#issuecomment-820484765
The goal of adding this line to the documentation is to point out the internal exception to the "MUST NOT contain any empty label names or values." rule and reinforce that it should not be relied upon anywhere else.
Just checked: So federation also adds an empty instance label implicitly (if there is no instance label already, i.e. pretty much what the PGW is doing). Not relevant here if you change the paragraph to what I have suggested, but it tells us that the federation documentation doesn't really explain that fact. So we should also update the federation documentation (over in prometheus/prometheus). It also makes the necessity to use an empty label in external_labels even rarer.
