Wiring ampmeter with reduced wire size
#20 wire is simply too small to carry 10A.  UL won't approve that.
It also won't meet boating codes.
So either this is a Chinese cheapie, or it is designed to use an ammeter shunt. (which on the negative side would be pretty safe).
An ammeter shunt is a really good idea.  Presumably you want that ammeter to be located somewhere useful, instead of between the panel and the charge controller.  You don't want the #12 power cable taking a long detour just to get the display where you want it - you want to keep the #12 cable runs as short as possible.  Voltage drop is a big deal when you're down at 12 volts.
With an ammeter shunt, you are free to put the ammeter where you want without worrying about that.