Use a plug for Prometheus metrics

The plug is injected as early as possible in order to respond quickly,
without polluting logs.

lib/music_library_web/endpoint.ex

@@ -15,6 +15,8 @@ defmodule MusicLibraryWeb.Endpoint do
     encrypt: true
   ]
 
+  plug MusicLibraryWeb.Telemetry.Plug, at: "/metrics"
+
   socket "/live", Phoenix.LiveView.Socket,
     websocket: [connect_info: [session: @session_options]],
     longpoll: [connect_info: [session: @session_options]]