I have a dehumidifier with pump that drains via a small plastic tubing
(about 5mm OD).
I'd like to monitor its performance by measuring how much water flows
through the tubing.
This should be cumulative. E.g. after one day the meter may indicate 1
gallon, after 2 days it may say 2 gallons, etc.

Is there a meter to do this? What is it called (so I can search for it)?