[Enhancement] Add Prometheus support (#556)

* Added prometheus to deps list

* WIP - screwing around with Prometheus and grafana

* Added basic prometheus config

* Updated docs in prom_ex module

* Updated README

lib/pinchflat_web/endpoint.ex

@@ -32,6 +32,8 @@ defmodule PinchflatWeb.Endpoint do
     plug Phoenix.Ecto.CheckRepoStatus, otp_app: :pinchflat
   end
 
+  plug PromEx.Plug, prom_ex_module: Pinchflat.PromEx
+
   plug Phoenix.LiveDashboard.RequestLogger,
     param_key: "request_logger",
     cookie_key: "request_logger"