Did you mean to measure the *current* flowing into a zone? I don’t think you can measure the *voltage* flowing into a zone — voltage is applied on the two wires of a zone.
I don’t quite understand how your pump works. So if the pump is cutoff, does it cut off the current flowing into it?
In any case, RPi does NOT have analog sensing pin (and OSPi no longer has analog-to-digital converter chip either). So as if there is no way for OSPi to sense current. You will need to get an external current sensing module.