Add telemetry event span for socket dispatch

[danschultzer]

The reason for adding this telemetry event is that my OTel spans are full of WebSocket connections that just reads GET. This is because Bandit will just use the method which is correct as the URL may be high cardinality, however the socket path is low cardinality and should show up as e.g. GET /live/websocket.

Since socket requests are handled and halted before the rest of the endpoint plugs I can't depend on Plug.Telemetry to report them and use the [:phoenix, :endpoint] events. I could depend on [:phoenix, :socket_connected] but this will only be for successful connections.

So I'm proposing that we add a [:phoenix, :socket_dispatch] event matching the [:phoenix, :router_dispatch] events. It somewhat conflicts with the existing [:phoenix, :socket_connected] though this event only have high level metadata vs socket connected having connection metadata including the result of setting up the socket.

With the ability to attach to this telemetry event we can update the :opentelemetry_phoenix to update the span name with the route template in the same way it currently does for router dispatch events.

[josevalim]

Hi @danschultzer. Our long term goal is to move the socket dispatch out of being hardcoded and make it part of the regular plug pipeline. This means we would eventually "break" this event. Therefore, if we want to automate it, I would do it for the upgrade command itself, if possible, otherwise we wait.