refactor: add typesafe wrappers for prom client metrics (#5969)

https://linear.app/unleash/issue/2-1856/add-typesafe-wrappers-over-prom-clients-metrics

As discussed on the latest knowledge sharing session, this adds typesafe
wrappers over prom client's metrics, requiring us to specify all the
configured labels for each metric.

This uses a functional approach and only exposes the methods that are
currently relevant to us, while also exposing the underlying instance of
the metric for an easy access if needed.

Since we often chain `labels` with `inc` in counters, this adds a
convenience `increment` method for counters which does both in a single
call.