Make FLP able to generate labels per aggregate

[KalmanMeth]

https://issues.redhat.com/browse/NETOBSERV-490

Currently, FLP merges all aggregate keys in an unique prom label called "aggregate".

It restricts what we can further query with promQL. To have full querying capabilities, we should have 1 label per aggregate.

E.g. for aggregates defined for SrcNamespace, DstNamespace, SrcSubnet, DstSubnet we should be able to get each of these as individual prom labels (instead of all concatenated in "by" and "aggregate" clauses).