Feature request: add support for Channels

[danielxvu]

In addition to the events already supported by this library, Phoenix supports the following ones as well:

    * `[:phoenix, :socket_connected]` - dispatched by `Phoenix.Socket`, 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, log: Logger.level | false}`
      * Disable logging: `use Phoenix.Socket, log: false` or `socket "/foo", MySocket, websocket: [log: false]` in your endpoint
    * `[:phoenix, :channel_joined]` - dispatched at the end of a channel join
      * Measurement: `%{duration: native_time}`
      * Metadata: `%{result: :ok | :error, params: term, socket: Phoenix.Socket.t}`
      * Disable logging: This event cannot be disabled
    * `[: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}`
      * Disable logging: This event cannot be disabled

Would you be willing to accept a pull request for this?

[aselder]

I'd love to see this, but without an event at the start of a handle_in, you can't get a span or trace started. We might want to talk to Phoenix team about adding an event at the start of handle_in.