Specify precisely the semantics of empty label values

[brian-brazil]

prometheus/docs#548

[brian-brazil]

This is getting a bit into Prometheus-specific semantics, but we should at least specify that an empty label value is the same as that label not existing.

[RichiH]

I would argue that this is not implementation-specific, as allowing either will lead to mismatching label sets in different ingestors; thus, it's a wire format consideration.

We do agree that 'empty' means "" only and not " "?

[brian-brazil]

Yes, the zero-length label value is empty - not any other value.