[NEW] Prometheus Backup Plugin

[thomasmitchell]

Is your feature request related to a problem? Please describe.
I want to back up Prometheus.

Describe the solution you'd like
The admin API of Prometheus has a snapshot endpoint that dumps an archive of the current data onto the box (similar to how Redis backups work).

https://prometheus.io/docs/prometheus/latest/querying/api/#snapshot

We should use that nice thing and make a plugin for it.

Describe alternatives you've considered

I could take a filesystem backup of TSDB, but Prometheus keeps putting data in there all the time for some reason and I can't take a temporally consistent snapshot with this method. I can't even guarantee that this doesn't generate a semantically-corrupted backup.