That is a good question that I've wondered myself. I don't think there is a ready-made software solution that currently exists, unfortunately. A cool idea for a project would be to build a current measuring device into the USB cable so that you can actively view the reading via a LCD or OLED display. It would just be to extract the functionality of the ...
All the Pi's GPIO are digital, i.e. on or off.
For the Pi to detect a voltage like 0.2 V the simplest solution would probably be to add an ADC (Analogue to Digital Converter).
Search for a popular ADC such as the MCP3008 for examples.
You will need to use additional analog circuitry.
A current is generally measured by running it through a shunt - a (low resistance) resistor of known value and applying Ohm's law to the detected voltage. Since you already say 0.2V, rather than some Amperage, I believe you have that part covered.
The next part is to convert it to a digital value. This can ...