Add with_prefix to Registry

[Victor-N-Suadicani]

Adds the ability to create a registry with a prefix.

This is actually already possible through the below hack but this makes it a lot easier and neater.

let prefixed_registry = {
    let mut registry = <Registry>::default();
    let prefixed_registry = registry.sub_registry_with_prefix(prefix);
    std::mem::take(prefixed_registry)
};

This is useful to set an application-wide prefix on the single registry for all your metrics.

[mxinden]

Just two comments. Thanks for sending in a pull request!

Would you mind adding a changelog entry?

[mxinden]

Great. Thanks!