H-Bridge driver: external current sensing
The pin needs a sense resistor (as shown in the diagram) and that value will be very low value circa tens of milliohm to prevent excessive power loss. You can interface that with an analogue pin on an MCU directly via a 1 kohm series resistor. The series resistor prevents any possibility of high current reaching the MCU should a fault develop in the DRV8870 and is low enough so that a standard MCU ADC input will work fine.

If you can live with the resolution you won't need a buffer amplifier. However, if your implemantion is a tad poor on earth plane you should consider using a differential amplifier with the diff amp reference 0 volts tied to analogue 0 volts at the MCU. You could use an instumentation amplifier too.