provide Prometheus metrics

[marten-seemann]

We should provide a way to collect Prometheus metrics for node operators.

The metrics tracer should probably be an implementation of the existing Tracer / ConnectionTracer. To make this performant, we'll need to resolve #3704 first. The code should live in a separate package, so that we're not forcing a Prometheus dependency onto anyone (there won't be a way to prevent it from being included in the go.mod though, unless we move this to a separate Go package).

We should also provide a sample Grafana dashboard.