I had this problem in Barcelona many years ago, where two pipes with different CP installations were in proximity (separate Owners). There is one aspect that may be worth consideration. If the body of the flowmeter is connected to the return side of its electronic circuit and the latter is not connected to the pipe itself, a parasitic current may flow, diverting some of the cathodic protection current.

Two solutions are (i) ensuring electrical isolation (floating) of the circuitry of the flowmeter and its external connections or (ii) ensuring that the return sides of the cathodic protection and the flowmeter are electrically tied together. Personally, I prefer the isolation, if the flowmeter design permits it.