protocols/gossipsub: Consider feature-gating the metrics support

[thomaseizinger]

Description

Currently, prometheus-client is a non-optional dependency of libp2p-gossipsub. Change it to an optional dependency, enabled by a feature flag (metrics f.e.)

Motivation

Not all users user the provided metric support. Making it optional reduces the dependency footprint.

Current Implementation

Are you planning to do it yourself in a pull request?

Maybe.