feat!: switch to metrics generator pattern

[wemeetagain]

Resolves #225

This will allow us to add metrics in a more agile way, since this library now maintains all code to create metrics.

This is a breaking change, since now, to enable metrics, instead of passing a IDiscv5Metrics object in the constructor, you pass a Registry object (and this library constructs the IDiscv5Metrics object)

Before:
Discv5.create({..., metrics})

After:
Discv5.create({..., metricsRegistry})

[dapplion]

I don't think discv5 should depend on prom-client directly. Instead just expect an specific interface that implements a metrics factory