Phoenix Channel handle_in Telemetry

[josia...@hifyre.com]

I am wondering if there is a specific reason that Phoenix channels only have a single [:phoenix, :channel_handled_in] telemetry event at the moment and no event to single the start of handle in. Comparing with the endpoint events or the live view events I would have expected the events to be [:phoenix, :channel, :handle_in, :start] and [:phoenix, :channel, :handle_in, :stop].

The reason I am looking for both start/stop telemetry events from the channel is that I would like to be able to hookup tracing with Spandex for channel interactions similarly to how Spandex Phoenix works to use telemetry events to trace endpoint requests. As far as I can tell the only other way to trace channel actions would be to start the trace at the beginning of every handle in function.

Since the current [:phoenix, :channel_handled_in] event couldn't be removed without likely breaking things for people, perhaps the start event could follow its naming convention to match. That, or a duplicate stop event could be added and the current one deprecated. Curious to hear other's thoughts on this, I would be happy to put up a PR if it seems like this is something that is wanted.