[nota]

water dept uses a set up like your original drawing on fire lines
to detect use of water from the fire line [illegal taps]

it is NOT used to measure flow volume just if there is flow at all

btw most pool filters have a pressure gauge on them
replace the gauge with a pressure switch to turn on at a given pressure [the 18 psi]
and hook the switch to light bulb
that way a simple look will tell if you need to clean the filter [backwash] if the light is on
no new pipes or fitting or leaks ect

guess you could add a low switch to detect a full pump basket also