Add monitoring (scraping + dashboard) for PaymentServer

This adds monitoring (Prometheus scraping) and a preliminary dashboard for the two PaymentServer HTTP endpoints.

Known issues:

1. The Grafana dashboard could be improved. I'd like to come back to this later and merge this MR as it is. What's included with this MR is much better than nothing.
2. There's no alerting yet. We need some experience in what's good thresholds first. And then we need to think how we want to be alerted.

This MR supersedes !92 (closed) and implements privatestorageops#300.

Here's how this looks: