The ADS1015 isn't exactly a voltmeter but it's close. It takes an input voltage between 0 and 4.096v and returns a 12-bit integer whose smallest increment is equivlent to 1mV.
If the voltage you want to measure is less than about 4v, you can use the ADS1015's output directly. If the voltage you want to measure is larger than 4v, you can put a voltage divider between that and GND, measure the voltage at the middle of the divider, then scale that up to get the actual voltage.
When you void a product warrany, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.