Easiest & smallest number of components would be something like the ACS product range from Allegro. ACS730 or 732 looks suitable, depending on if you need to measure bidirectional or unidirectional only.
A shunt & opamp system would work as well, but takes * a lot * more components to get near the same performance as e.g. an ACS730 and a whole lot of engineering. You can of course always do a very simple/minimal design with just a shunt, a single opamp and the ADC on an Arduino but performance won't be close to the ready-made products mentioned and you'll either battle noise (very small shunt) or poor efficiency (larger shunt).