Assuming that your baby is growing well and doesn't have any problems that you're aware of, the closer your baby is to full term (40 weeks) the lower the risk of any complications from prematurity. Although the survival rate is excellent even at 32 weeks, the risk for problems from prematurity exist all the way up until 39 - 40 weeks.
Although the use (or discontinuation) of metformin is not really associated with prematurity, you should discuss with your doctor the risks vs. benefits of stopping the metformin. One other consideration is whether stopping the metformin may have any effect on your blood sugars. Only your doctor knows enough about your medical condition and history to advise you about the possibility of any problems if you stop.