I was on metformin up until week 12 of the pg, but not since. I didn't have issues with supply.

FJ's link re medications & breastfeeding has an article on metformin. Medications & Breastfeeding. Just scroll down to Metformin Transfer into Human Milk

Apparently it does go through to the milk, but they seem to conclude that women taking Metformin for PCOS or diabetes should still be encouraged to breastfeed.