metrics: add options to disable metrics and to set Prometheus registerer

[sukunrt]

fixes: #2047

[sukunrt]

I made an implementation with generics too. But that seems too complex for 50 lines of code reuse.
It's in my branch: sukunrt:metrics-registerer-generic

[marten-seemann]

Agreed to not make this generic.

There now needs to be some way to set the Registerer via a libp2p option.

[marten-seemann]

Probably slightly safer to do use errors.Is

[sukunrt]

the returned error is an instance of the type

type AlreadyRegisteredError struct {
	ExistingCollector, NewCollector Collector
}

Checking with errors.Is will not work since we do not know the existing collector, so I changed this to use errors.As

There is a tricky case here:
If an error is returned where we are registering a collector where a different collector with the same fully qualified name was registered before, this method will suppress it and the collector used as argument to the method will not be collected for metrics. In this example:

	reg := prometheus.NewRegistry()
	c1 := prometheus.NewCounter(
		prometheus.CounterOpts{
			Name: "counter",
		},
	)
	c2 := prometheus.NewCounter(
		prometheus.CounterOpts{
			Name: "counter",
		},
	)
	RegisterCollectors(reg, c1)
    RegisterCollectors(reg, c2) // c2 is not registered
    c1.Inc()
    c2.Inc() // This will have no effect

I'm leaving it as it is because this situation doesn't arise anywhere in the code yet since we are using namespaces for our metrics