Consider switching to the official Prometheus crate for metrics

[koute]

We're currently using the prometheus crate to gather our metrics. But there actually exists an official Prometheus crate which has a bunch of improvements over what we're using right now. This includes better performance (and metric gathering does show up when profiling) and would make things like this unnecessary.

Another argument for the switch is that libp2p also uses the official crate for its metrics, so we have this crate in our tree anyway.