Why do you feel you need a flow meter?
It adds flow restriction to the loop and is really not necessary, it is more of a novelty than a necessity.
I've never ever run a flow meter and I'm running dual loops, most water cooling veterans don't run one.
If your water pump dies you'll know it, your CPU or GPUs will simply shut down, replace the bad water pump and you're back in business.
When you add unnecessary flow restriction to your pump load, you shorten the pump life.