Add system monitor support

[mrkirby153]

Adds support for receiving system monitor events from :erlang.system_monitor.

There are two emitters by default:

• An emitter that emits metrics for events
• An emitter that logs events to the standard logger.

[ollien]

If one pid calls subscribe more than once, ref will end up being a list with more than one element, and no unsubscription will be performed here. I would rewrite this function to something like

  state = 
    state.subscribers
    |> Enum.filter(state.subscribers, fn {_, p} -> p == pid end)
    |> Enum.reduce(state, fn {ref, _pid}, state ->
        Process.demonitor(ref)
        %__MODULE__{state | subscribers: Map.delete(state.subscribers, ref)
    end)

{:reply, :ok, state}

[mrkirby153]

It's not possible for pid to be in the map more than once, subscribe/1 checks if the pid is already subscribed, if it is, it no-ops.

[ollien]

Ah you're right. I still think it's a bit cleaner to handle it as a list still but that's just my 2c. Won't block on it