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

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]
 

lib/music_library_web/telemetry.ex

@@ -10,7 +10,7 @@ defmodule MusicLibraryWeb.Telemetry do
   def init(_arg) do
     children = [
       {MusicLibraryWeb.Telemetry.Storage, metrics()},
-      {TelemetryMetricsPrometheus, [metrics: prometheus_metrics()]},
+      {TelemetryMetricsPrometheus.Core, [metrics: prometheus_metrics()]},
       # Telemetry poller will execute the given period measurements
       # every 10_000ms. Learn more here: https://hexdocs.pm/telemetry_metrics
       {:telemetry_poller, measurements: periodic_measurements(), period: 10_000}