feat: display the count of connected tenants in metrics

lib/realtime/monitoring/prom_ex.ex

@@ -1,5 +1,5 @@
 defmodule Realtime.PromEx do
-  alias Realtime.PromEx.Plugins.{OsMon, Phoenix}
+  alias Realtime.PromEx.Plugins.{OsMon, Phoenix, Tenants}
 
   @moduledoc """
   Be sure to add the following to finish setting up PromEx:
@@ -70,7 +70,8 @@ defmodule Realtime.PromEx do
       # {Plugins.Ecto, poll_rate: poll_rate, metric_prefix: [:ecto]},
       # Plugins.Oban,
       # Plugins.PhoenixLiveView
-      {OsMon, poll_rate: poll_rate}
+      {OsMon, poll_rate: poll_rate},
+      {Tenants, poll_rate: poll_rate}
     ]
   end
 

lib/realtime/monitoring/tenants.ex

@@ -0,0 +1,50 @@
+defmodule Realtime.PromEx.Plugins.Tenants do
+  @moduledoc false
+
+  use PromEx.Plugin
+  require Logger
+
+  @event_connected [:prom_ex, :plugin, :tenants, :connected]
+
+  @impl true
+  def polling_metrics(opts) do
+    poll_rate = Keyword.get(opts, :poll_rate)
+
+    [
+      metrics(poll_rate)
+    ]
+  end
+
+  defp metrics(poll_rate) do
+    Polling.build(
+      :realtime_tenants_events,
+      poll_rate,
+      {__MODULE__, :execute_metrics, []},
+      [
+        last_value(
+          [:tenants, :connected],
+          event_name: @event_connected,
+          description: "The total count of connected tenants.",
+          measurement: :connected
+        )
+      ]
+    )
+  end
+
+  def execute_metrics() do
+    connected =
+      if Enum.member?(:syn.node_scopes(), Extensions.PostgresCdcRls) do
+        :syn.local_registry_count(Extensions.PostgresCdcRls)
+      else
+        -1
+      end
+
+    execute_metrics(@event_connected, %{
+      connected: connected
+    })
+  end
+
+  defp execute_metrics(event, metrics) do
+    :telemetry.execute(event, metrics, %{})
+  end
+end