Refactor `RateLimit` Collector and add a proper `Description` for prom crate

[njasm]

The prometheus crate we use hashes all the metrics information (namespace and labels) from all registered Collectors and will error out if the same hash was previously registered.

this information is gathered by the crate when calling the desc method of the Collector trait, and right now our implementation of that is returning an empty vec

context see: #15 (comment)_

[njasm]

I'll work on this one as soon as I finish #15