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/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