merge prometheus2 targets into prometheus1

I am not exactly sure what that means exactly, but prometheus1 needs to supplant prometheus2 so the latter can be retired and replaced with a replica that's highly available.

I think it means we need to deploy the prometheus-alerts repository onto the prometheus1 server. We might also need to do something about all those dashboards in Grafana.

We'll run both servers in parallel for a year (or whatever the prometheus2 retention period is) and then retire prometheus2.

As usual, notify users and all.