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

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]]

lib/music_library_web/router.ex

@@ -69,10 +69,6 @@ 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/plug.ex

@@ -0,0 +1,22 @@
+defmodule MusicLibraryWeb.Telemetry.Plug do
+  @behaviour Plug
+
+  import Plug.Conn
+
+  def init(opts) do
+    Keyword.validate!(opts, [:at])
+  end
+
+  def call(conn, opts) do
+    at = Keyword.fetch!(opts, :at)
+
+    if conn.request_path == at do
+      conn
+      |> put_resp_content_type("text/plain")
+      |> send_resp(200, TelemetryMetricsPrometheus.Core.scrape())
+      |> halt()
+    else
+      conn
+    end
+  end
+end