add :telemetry for clients

[the-mikedavis]

instrumentation with telemetry could be valuable, but I'm not sure what events we would want to emit

what questions would we be trying to answer with telemetry?

[the-mikedavis]

see the guide from Phoenix.Logger: https://hexdocs.pm/phoenix/Phoenix.Logger.html

• [:phoenix, :socket_connected] - dispatched at the end of a socket connection
  • Measurement: %{duration: native_time}
  • Metadata: %{endpoint: atom, transport: atom, params: term, connect_info: map, vsn: binary, user_socket: atom, result: :ok | :error, serializer: atom}
• [:phoenix, :channel_joined] - dispatched at the end of a channel join
  • Measurement: %{duration: native_time}
  • Metadata: %{params: term, socket: Phoenix.Socket.t}
• [:phoenix, :channel_handled_in] - dispatched at the end of a channel handle in
  • Measurement: %{duration: native_time}
  • Metadata: %{event: binary, params: term, socket: Phoenix.Socket.t}

[the-mikedavis]

~there's probably a good argument to make for splitting out (by event type) telemetry for the connection process and telemetry for the client~

see #13

this issue now only pertains to the client's telemetry

[the-mikedavis]

this is really probably not as hard as I would imagine off the bat...

• store connection_id & join_ids in the socket (non-assigns area, maybe a new field called :metadata)
  • those values get hydrated by the Slipstream.Socket.apply_event/? function for ChannelConnected and TopicJoinSucceeded events
• don't support the await_* interface
• can probably take measurements in the Slipstream.Callback.dispatch/? function

still some open questions that can be revisited later:

• make a distinction between messages and replies?
  • probably put a flag in the metadata, don't make a whole separate event name
• should the equivalent for [:phoenix, :channel_joined] dispatch at the end of the c:Slipstream.handle_joined/3 callback or at the associated TopicJoinClosed/ChannelClosed event?
  • probably the c:handle_joined/3 is fine
• stuff for other callbacks?
  • punt!