Add prometheus client library to labkit-ruby

The oficial client https://github.com/prometheus/client_ruby is not performant enough, reason why we opted to forking into https://gitlab.com/gitlab-org/ruby/gems/prometheus-client-mmap that uses a custom memory mapped file implementation to share metrics among multiple processes.

We should move prometheus-client-mmap from the monolith to labkit-ruby in order to support metric emitting feature and maintain consistency across different ruby apps.

LabKit should include a wrapper around the features of prometheus-client-mmap, hiding the internals and facilitating an eventual migration to another client in the future.