Keep telemetry metrics history

lib/music_library_web/router.ex

@@ -67,6 +67,7 @@ defmodule MusicLibraryWeb.Router do
 
       live_dashboard "/dashboard",
         metrics: MusicLibraryWeb.Telemetry,
+        metrics_history: {MusicLibraryWeb.Telemetry.Storage, :metrics_history, []},
         ecto_repos: [MusicLibrary.Repo]
     end
   end

lib/music_library_web/telemetry.ex

@@ -9,6 +9,7 @@ defmodule MusicLibraryWeb.Telemetry do
   @impl true
   def init(_arg) do
     children = [
+      {MusicLibraryWeb.Telemetry.Storage, 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}

lib/music_library_web/telemetry/storage.ex

@@ -0,0 +1,66 @@
+defmodule MusicLibraryWeb.Telemetry.Storage do
+  use GenServer
+
+  @history_buffer_size 50
+
+  def metrics_history(metric) do
+    GenServer.call(__MODULE__, {:data, metric})
+  end
+
+  def start_link(args) do
+    GenServer.start_link(__MODULE__, args, name: __MODULE__)
+  end
+
+  @impl true
+  def init(metrics) do
+    Process.flag(:trap_exit, true)
+
+    metric_histories_map =
+      metrics
+      |> Enum.map(fn metric ->
+        attach_handler(metric)
+        {metric, CircularBuffer.new(@history_buffer_size)}
+      end)
+      |> Map.new()
+
+    {:ok, metric_histories_map}
+  end
+
+  @impl true
+  def terminate(_, metrics) do
+    for metric <- metrics do
+      :telemetry.detach({__MODULE__, metric, self()})
+    end
+
+    :ok
+  end
+
+  defp attach_handler(%{event_name: name_list} = metric) do
+    :telemetry.attach(
+      {__MODULE__, metric, self()},
+      name_list,
+      &__MODULE__.handle_event/4,
+      metric
+    )
+  end
+
+  def handle_event(_event_name, data, metadata, metric) do
+    if data = Phoenix.LiveDashboard.extract_datapoint_for_metric(metric, data, metadata) do
+      GenServer.cast(__MODULE__, {:telemetry_metric, data, metric})
+    end
+  end
+
+  @impl true
+  def handle_cast({:telemetry_metric, data, metric}, state) do
+    {:noreply, update_in(state[metric], &CircularBuffer.insert(&1, data))}
+  end
+
+  @impl true
+  def handle_call({:data, metric}, _from, state) do
+    if history = state[metric] do
+      {:reply, CircularBuffer.to_list(history), state}
+    else
+      {:reply, [], state}
+    end
+  end
+end

mix.exs

@@ -59,6 +59,7 @@ defmodule MusicLibrary.MixProject do
        depth: 1},
       {:telemetry_metrics, "~> 1.0"},
       {:telemetry_poller, "~> 1.0"},
+      {:circular_buffer, "~> 0.4.1"},
       {:gettext, "~> 0.20"},
       {:jason, "~> 1.2"},
       {:dns_cluster, "~> 0.1.1"},

mix.lock

@@ -2,6 +2,7 @@
   "bandit": {:hex, :bandit, "1.6.0", "9cb6c67c27cecab2d0c93968cb957fa8decccb7275193c8bf33f97397b3ac25d", [:mix], [{:hpax, "~> 1.0", [hex: :hpax, repo: "hexpm", optional: false]}, {:plug, "~> 1.14", [hex: :plug, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}, {:thousand_island, "~> 1.0", [hex: :thousand_island, repo: "hexpm", optional: false]}, {:websock, "~> 0.5", [hex: :websock, repo: "hexpm", optional: false]}], "hexpm", "fd2491e564a7c5e11ff8496ebf530c342c742452c59de17ac0fb1f814a0ab01a"},
   "castore": {:hex, :castore, "1.0.10", "43bbeeac820f16c89f79721af1b3e092399b3a1ecc8df1a472738fd853574911", [:mix], [], "hexpm", "1b0b7ea14d889d9ea21202c43a4fa015eb913021cb535e8ed91946f4b77a8848"},
   "cc_precompiler": {:hex, :cc_precompiler, "0.1.10", "47c9c08d8869cf09b41da36538f62bc1abd3e19e41701c2cea2675b53c704258", [:mix], [{:elixir_make, "~> 0.7", [hex: :elixir_make, repo: "hexpm", optional: false]}], "hexpm", "f6e046254e53cd6b41c6bacd70ae728011aa82b2742a80d6e2214855c6e06b22"},
+  "circular_buffer": {:hex, :circular_buffer, "0.4.1", "477f370fd8cfe1787b0a1bade6208bbd274b34f1610e41f1180ba756a7679839", [:mix], [], "hexpm", "633ef2e059dde0d7b89bbab13b1da9d04c6685e80e68fbdf41282d4fae746b72"},
   "db_connection": {:hex, :db_connection, "2.7.0", "b99faa9291bb09892c7da373bb82cba59aefa9b36300f6145c5f201c7adf48ec", [:mix], [{:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "dcf08f31b2701f857dfc787fbad78223d61a32204f217f15e881dd93e4bdd3ff"},
   "decimal": {:hex, :decimal, "2.2.0", "df3d06bb9517e302b1bd265c1e7f16cda51547ad9d99892049340841f3e15836", [:mix], [], "hexpm", "af8daf87384b51b7e611fb1a1f2c4d4876b65ef968fa8bd3adf44cff401c7f21"},
   "dns_cluster": {:hex, :dns_cluster, "0.1.3", "0bc20a2c88ed6cc494f2964075c359f8c2d00e1bf25518a6a6c7fd277c9b0c66", [:mix], [], "hexpm", "46cb7c4a1b3e52c7ad4cbe33ca5079fbde4840dedeafca2baf77996c2da1bc33"},