[Enhancement][ActionCable] Add ActiveSupport notifications to open, close, subscribed and unsubscribed actions

Looks like some ActionCable actions are missing ActiveSupport notifications. Now that we have e.g. “perform_action” and “broadcast” instrumented, I suppose it’d be good to do the same for other important calls, namely: “open”, “close”, “subscribed” and “unsubscribed”. I have this implemented in my fork: https://github.com/xlts/rails/commit/2d9bb3197e2d3c7a522611c3ec5b778442c30362. Would it be reasonable to submit a PR?