Remove Prometheus-based metrics

Never use them

lib/music_library_web/endpoint.ex

@@ -15,8 +15,6 @@ 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/telemetry.ex

@@ -10,7 +10,6 @@ defmodule MusicLibraryWeb.Telemetry do
   def init(_arg) do
     children = [
       {MusicLibraryWeb.Telemetry.Storage, 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}
@@ -104,39 +103,6 @@ defmodule MusicLibraryWeb.Telemetry do
     ]
   end
 
-  def prometheus_metrics do
-    [
-      # VM Metrics
-      last_value("vm.memory.total", unit: {:byte, :megabyte}),
-      last_value("vm.total_run_queue_lengths.total"),
-      last_value("vm.total_run_queue_lengths.cpu"),
-      last_value("vm.total_run_queue_lengths.io"),
-
-      # Database Metrics
-      last_value("music_library.repo.query.total_time",
-        unit: {:native, :millisecond},
-        description: "The sum of the other measurements"
-      ),
-      last_value("music_library.repo.query.decode_time",
-        unit: {:native, :millisecond},
-        description: "The time spent decoding the data received from the database"
-      ),
-      last_value("music_library.repo.query.query_time",
-        unit: {:native, :millisecond},
-        description: "The time spent executing the query"
-      ),
-      last_value("music_library.repo.query.queue_time",
-        unit: {:native, :millisecond},
-        description: "The time spent waiting for a database connection"
-      ),
-      last_value("music_library.repo.query.idle_time",
-        unit: {:native, :millisecond},
-        description:
-          "The time the connection spent waiting before being checked out for the query"
-      )
-    ]
-  end
-
   defp periodic_measurements do
     [
       # A module, function and arguments to be invoked periodically.

lib/music_library_web/telemetry/plug.ex

@@ -1,22 +0,0 @@
-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