Metrics should be able to accept a prometheus registerer parameter

[MarcoPolo]

Instead of always using the default registerer.

[sukunrt]

Should I add An option to create a metrics tracer with a given registrar like done in eventbus metrics?
https://github.com/libp2p/go-libp2p/blob/master/p2p/host/eventbus/basic_metrics.go#L96-L100

func MustRegisterWith(reg prometheus.Registerer) MetricsTracerOption {
	return func(s *metricsTracerSetting) {
		s.reg = reg
	}
}

[marten-seemann]

I'd be in favor of renaming the option. MustRegisterWith sounds like it's a global function (which it isn't really, it's an option), and it also sounds like it could panic. Better: WithRegisterer.

If I understand correctly, when allowing multiple prometheus.Registerers, we need to be careful with the sync.Once. We need to register once with every Registerer.

We probably need to introduce some kind of metrics helper: A (thread-safe) set to keep track of which Registerers we've already registered with.

[sukunrt]

Agreed on WithRegisterer.
Why do we need to register just once? The prometheus Registerer interface has a Register method which would give an error AlreadyRegisteredError on reregistration. We can ignore this error and panic/propagate other errors when doing registerer.Register. Is this good enough?

[marten-seemann]

Why do we need to register just once? The prometheus Registerer interface has a Register method which would give an error AlreadyRegisteredError on reregistration. We can ignore this error and panic/propagate other errors when doing registerer.Register. Is this good enough?

Looks like I didn't look closely enough at the Prometheus API. This is even better! Catching AlreadyRegisteredError and panicking for all others sounds reasonable to me.