An electrical submersible pump assembly has a centrifugal primary pump having a primary pump housing. An electrical motor is operatively coupled to the primary pump. An instrument sub has a housing secured by threaded fasteners to one end of the primary pump housing. A pump stage in the instrument sub housing is driven by the electric motor along with the primary pump, the pump stage including an impeller and a diffuser. A sensor within the instrument sub monitors parameters in the instrument sub. A sensor line port extends through a wall of the instrument sub housing. A sensor line extends sealingly through the sensor line port to the sensor for conveying sensed information from the sensor to a remote instrument panel.