From b111558f074c53bc5f03b6e91ad1a00c96958ead Mon Sep 17 00:00:00 2001
From: Claudio Ortolina <cloud8421@gmail.com>
Date: Tue, 4 Mar 2025 20:21:15 +0000
Subject: [PATCH] Use a plug for Prometheus metrics

The plug is injected as early as possible in order to respond quickly,
without polluting logs.
---
 .../controllers/metrics_controller.ex         |  7 ------
 lib/music_library_web/endpoint.ex             |  2 ++
 lib/music_library_web/router.ex               |  4 ----
 lib/music_library_web/telemetry/plug.ex       | 22 +++++++++++++++++++
 4 files changed, 24 insertions(+), 11 deletions(-)
 delete mode 100644 lib/music_library_web/controllers/metrics_controller.ex
 create mode 100644 lib/music_library_web/telemetry/plug.ex

diff --git a/lib/music_library_web/controllers/metrics_controller.ex b/lib/music_library_web/controllers/metrics_controller.ex
deleted file mode 100644
index 4b68a5bb..00000000
--- a/lib/music_library_web/controllers/metrics_controller.ex
+++ /dev/null
@@ -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
diff --git a/lib/music_library_web/endpoint.ex b/lib/music_library_web/endpoint.ex
index 17ef1de4..ca75e953 100644
--- a/lib/music_library_web/endpoint.ex
+++ b/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]]
diff --git a/lib/music_library_web/router.ex b/lib/music_library_web/router.ex
index fedd15c6..b078cb9e 100644
--- a/lib/music_library_web/router.ex
+++ b/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]
 
diff --git a/lib/music_library_web/telemetry/plug.ex b/lib/music_library_web/telemetry/plug.ex
new file mode 100644
index 00000000..9625eca9
--- /dev/null
+++ b/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