Unlike other sensors like rain or soil sensor, flow sensor will generate a large number of clicks (it can be tens or even hundreds per second), it’s not feasible to send notifications each time the flow sensor clicks. I don’t think it’s a good idea to send sensor1 events if sensor 1 is configured as flow. Instead, the current approach is that it will send one flow sensor notification when a zone completes running.
If you want to sample and send flow sensor values, that’s fine. It’s not currently implemented in the firmware but it can be easily added. Again, it should not be very frequently otherwise you will receive a large number of notifications which is not ideal.