Mount metrics in application

Avoids pulling entire TelemetryMetricsPrometheus dependency.

lib/music_library_web/router.ex

@@ -69,6 +69,10 @@ defmodule MusicLibraryWeb.Router do
     use ErrorTracker.Web, :router
     import Phoenix.LiveDashboard.Router
 
+    scope "/metrics", MusicLibraryWeb do
+      get "/", MetricsController, :index
+    end
+
     scope "/dev" do
       pipe_through [:browser, :logged_in]