It would be nice to have an ActiveSupport::Notifications instrumentation hook for ActionCable::Channel::Base#perform_action similar to the one for ActionController#process_action.
I have implemented this in my fork: https://github.com/mwear/rails/commit/c8f5b5ecdc6a89493e4de1bb1b7b865a70af2151 and would be happy to open a PR if others would find this useful.