simulate this circuit – Schematic created using CircuitLab
Figure 1. Relay isolation.
I would first attempt to find a quick-acting 24 V AC relay, wire that in parallel with the reel solenoid and use a debounced contact of the relay to drive the micro-controller GPIO with internal pull-up enabled.
Figure 2. A selection of 24VAC+relays found in an image search.
If your counter reel can react to the shortest pulse then your mechanical relay should be able to as well. I would be inclined to mount the relay close to the micro and run a twisted pair of wires to the coil to minimise any coupling between the AC wiring and the logic signals.