[Feature] Allow custom labels to be set

[NickLarsenNZ]

I'd like to be able to see the DB host as a metrics label (eg: taken from the DSN). Not sure how easy that is given the various formats of DSNs. Would it be possible?

[mymasse]

Or allowing a custom label to be added to all metrics. We plan on spin up multiple containers in our Kubernetes cluster one per database, but right now there is no way to know which one is which, my plan is to currently give the containers a database specific name but having the DB host would be better