You might talk to your doctor about cutting back your metformin if you're going to work out regularly.
The most efficient way for anyone to lose weight, but especially for a diabetic is to cut calories. Working out is good for your overall health, but you'll often have to eat as much glucose as you burn for the first few weeks of working out.
When you first start a new workout it can substantially drop your blood sugar. But after your muscles become more efficient it takes less glucose and the blood sugar drop is less.
So that means for the first few weeks of working out you'll have lower blood sugars and need to eat glucose tablets to keep blood sugar up.
There's a good book called "The Diabetic Athlete" by Sheri Colberg who's a longtime type 1 diabetic. You might be able to find it at the library. I like that it gives advice on how to eat to cover the carbs expelled through exercise.