[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

config/dev.exs

@@ -81,3 +81,5 @@ config :phoenix_live_view, :debug_heex_annotations, true
 
 # Disable swoosh api client as it is only required for production adapters.
 config :swoosh, :api_client, false
+
+config :pinchflat, Pinchflat.PromEx, disabled: false