[analyse] Feature Request: Multiple Prometheus-compatible backend support

[eamonryan]

Right now, analyse only supports a single Prometheus style backend under cortextool analyse prometheus

However, it's not uncommon for someone to have many Prometheus backends connected to a single Grafana instance - multiple Prometheus servers, or Thanos clusters, or what have you.

So, it would be helpful for analyse to support a list/array of Prometheus backends to diff a Grafana instance against for unused metrics, since checking just one backend gives an incomplete picture, and checking multiple backends individually with the tool as it is today requires stitching all the results together somehow.

cc @hjet