add nodes_connected metric (#1517)

lib/archethic/p2p.ex

@@ -102,6 +102,26 @@ defmodule Archethic.P2P do
     end
   end
 
+  @doc """
+  Called by the telemetry poller
+  """
+  def nodes_connected_count() do
+    nodes_connected =
+      authorized_and_available_nodes()
+      |> Enum.reject(&(&1.first_public_key == Crypto.first_node_public_key()))
+      |> Enum.filter(&node_connected?/1)
+      |> Enum.count()
+
+    :telemetry.execute(
+      [:archethic, :p2p],
+      %{nodes_connected: nodes_connected}
+    )
+  rescue
+    _ ->
+      # this will fail at startup because ETS table does not exist yet
+      :ok
+  end
+
   @doc """
   List the nodes registered.
   """

lib/archethic/telemetry.ex

@@ -18,7 +18,8 @@ defmodule Archethic.Telemetry do
 
   defp periodic_metrics do
     [
-      {Archethic.Contracts, :maximum_calls_in_queue, []}
+      {Archethic.Contracts, :maximum_calls_in_queue, []},
+      {Archethic.P2P, :nodes_connected_count, []}
     ]
   end
 
@@ -108,6 +109,7 @@ defmodule Archethic.Telemetry do
           buckets: [500, 700, 1000, 1500, 2000, 3000, 5000, 10000]
         ]
       ),
+      last_value("archethic.p2p.nodes_connected"),
       distribution("archethic.p2p.send_message.duration",
         unit: {:native, :millisecond},
         measurement: :duration,