This circuit is measuring the voltage drop at Rs, which is propotional to the current that flows through it an hence through the load (since they are in series). With a large input impedance in the op-amp, no significant current will flow to it so the previous sentence is correct. Yes, the op-amp output voltage will be proportional to current. You just need do add an A/D converter and then feed this to the microcontroller. You can choose a microcontroller with an A/D converter embedded (there are lots of them) which will simplify your work.