touch sensor for metal door handle
You should do some research into touch sensors and touch sensor circuits. There are even some microcontroller manufacturers that are offering touch sensing application notes and some even include the touch sensor circuitry right on the MCU. 

You want to concentrate on capacitive touch sensor technology as it does not depend upon a direct current path through the human body. Instead it depends upon the capacitive change in the circuit when a human touches the sensor. 

If the circuit is configured correctly you may very well get your metal door knob to work as a touch sensor. It would likely not work so well if the knob is mounted in a metal or metal clad door.