I've never experienced the itching with metformin. Have you contacted your physician about this?
Is metformin the only thing that has changed in your routine since the itching started? Are you still using the same detergent, soap, shampoo, dryer sheets, etc? Is the itching all over or confined to a specific area?
Have you tried an OTC med such as Benadryl, Claritin or such to see if this will break the itch cycle?
Are you keeping your skin well-moisturized?