Puma metrics middleware

[mhoofman]

As described in the added README.md documentation:

This allows for an alternative method to collect metrics. The middleware fetches stats from the Puma control app and parses them into Prometheus metrics. This is useful when your application already provides a metrics endpoint.

For our use case, we wanted a way to integrate the Puma metrics into an already existing /metrics endpoint that was already set up and being scraped by Prometheus.

This middleware should work for any rack-based applications. This was tested with a Rails application.

I look forward to any feedback. Thanks for your consideration.