That would be fantastic. Calories, heart rate, etc. aren't really that important to me when I'm lifting, which is my favorite form of exercise.
But I wonder how Apple or any other wearable OEM could track it. I guess they could work with Weider and other machine manufacturers to embed sensors that track weight, reps, etc. and then send that Watch. I use only free weights, and I don't know how that could be tracked aside from a sensor and transmitter in every plate, dumbbell and bar that I own.