I've had this set on previous Lumix cameras but I can't remember how I did it. :-)
I want to set the lower custom function button on the front of the body to be Spot Metering as soon as I press the button down (specifically spot metering, not bring up the metering menu).... but ONLY be spot metering while I'm holding the button down. So, press and hold for temporary spot metering, then release and I'm back to the metering mode I normally have selected in the menus.
Any ideas? I also do this on my Nikon Z8. It's very convenient when you only need spot metering once in a while and only for a shot or two.