Mount metrics in application

Avoids pulling entire TelemetryMetricsPrometheus dependency.

lib/music_library_web/controllers/metrics_controller.ex

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