[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/runtime.exs

@@ -72,6 +72,7 @@ if config_env() == :prod do
   # For running PF in a subdirectory via a reverse proxy
   base_route_path = System.get_env("BASE_ROUTE_PATH", "/")
   enable_ipv6 = String.length(System.get_env("ENABLE_IPV6", "")) > 0
+  enable_prometheus = String.length(System.get_env("ENABLE_PROMETHEUS", "")) > 0
 
   config :logger, level: String.to_existing_atom(System.get_env("LOG_LEVEL", "debug"))
 
@@ -95,6 +96,8 @@ if config_env() == :prod do
     database: db_path,
     journal_mode: journal_mode
 
+  config :pinchflat, Pinchflat.PromEx, disabled: !enable_prometheus
+
   # The secret key base is used to sign/encrypt cookies and other secrets.
   # A default value is used in config/dev.exs and config/test.exs but you
   # want to use a different value for prod and you most likely don't want