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/controllers/metrics_controller.ex

@@ -1,7 +0,0 @@
-defmodule MusicLibraryWeb.MetricsController do
-  use MusicLibraryWeb, :controller
-
-  def index(conn, _params) do
-    send_resp(conn, 200, TelemetryMetricsPrometheus.Core.scrape())
-  end
-end