Add global labels to all timeseries.

Due to the structure of targetmanager/targetpool/target and who has visiblity
of which data, this is a bit ugly in places (like upon target add/replace,
getting each targets' existing labels, merging in the global ones, storing them
back).  But I doubt it can be done much nicer without completely refactoring
the retrieval code, which we should do at some point.