OP:
Torque is a measurement of power from the engine, at the gear box of the prop/rotor.
The reason for torque indication is the gear box has limitations on how much power it can handle, as it's being resisted on the other side by the prop/rotor, which is being resisted by the drag from the air.
Certainly you can measure torque of the engine, torque to the prop, but usually the gear box is the weakest link, as it's translating the power from the engine to the airfoil...so it usually gets the indicator, with your job keeping it out of the red.