Feature Request - Add option to acquire labels/values from Prometheus API

[heapdavid]

Thanks for your work on Karma.

It would be handy to be able to optionally include labels and their values from Prometheus in the create/edit silence dialogue, for planning silences in advance where the labels required are not yet in Alertmanager's list.

The Prometheus HTTP API looks to be pretty straightforward: https://prometheus.io/docs/prometheus/latest/querying/api/#querying-metadata

Could see it being a (default off) toggle switch or tickbox on the create/edit silence dialogue.

[ceccillia]

This would be a nice addition