I am actually working on some similar stuff in the PromEx Project GitHub - akoutmos/prom_ex: An Elixir Prometheus metrics collection library built on top of Telemetry with accompanying Grafana dashboards · GitHub

At the moment I am using the [:phoenix, :channel_joined] telemetry event to capture metrics on how many channel joins have occurred but that won’t tell you how many people are currently attached to a channel as I don’t keep track of when people leave the channel. As for the channel that the joined event is in regards to, that is in the event metadata under the Phoenix.Socket struct:

event_metadata = %{
  params: %{},
  result: :ok,
  socket: %Phoenix.Socket{
    assigns: %{},
    channel: Phoenix.LiveReloader.Channel,
    channel_pid: #PID<0.633.0>,
    endpoint: WebAppWeb.Endpoint,
    handler: Phoenix.LiveReloader.Socket,
    id: nil,
    join_ref: "6",
    joined: false,
    private: %{log_handle_in: :debug, log_join: :info},
    pubsub_server: WebApp.PubSub,
    ref: nil,
    serializer: Phoenix.Socket.V2.JSONSerializer,
    topic: "phoenix:live_reload",
    transport: :websocket,
    transport_pid: #PID<0.630.0>
  }
}

Hope that helps!

Someone can correct me here, but I imagine the issue with tracking leaves is that the disconnection event can’t be guaranteed depending on how the process is killed. That may be wrong depending on supervision setup, but my immediate thought goes to some edge cases that prevent complete guarantees.

I assume you’re correct if you are wanting to track disconnects from within the process that is “dying”. That’s why I am monitoring the channel process separately.