Hi Alex,
You say on your profile you have pre-diabetes, so at the moment you haven't got diabetes but may be heading that way. Do you know what your blood glucose tests were when your doctor told you that you are pre-diabetic? How long ago was that, and have you had any other tests?
I'm afraid that diabetes is random. It affects thin people as well as fat people, it's just that fatter people are more prone to it.
Metformin is the first line medication for diabetics, though not all of us are on any medication at all, me included. It helps with certain things, but only to a limited extent and is mainly an appetite suppressant to help people lose weight. I can't believe your doctor prescribed it for a pre-diabetic that is normal weight, but there you go. I'm not a doctor.
Personally I am a Type 2, diagnosed in January this year, no medication. I control it with diet and exercise, but not a lot of exercise! I followed the advice given by many people on this forum to eat a low carb diet, cut out all the obvious culprits like sweets, sugar and what not, eat 3 meals a day as evenly spaced as I can manage, and I learnt what foods my body can cope with by buying a home testing meter and testing my blood levels before eating then at 1 hour and 2 hours after eating. This way you can see just what that meal has done to your levels, then you learn from it. You either avoid the food that caused levels to rise in future, or reduce the portions until the portions are at a level that you can manage. You can eat as much meat, fish, eggs, green veggies, mushrooms, cheese, butter etc as you like. What you need to be careful of are potatoes, rice, pasta and bread, plus certain fruits. If you eat like that, the cravings will eventually go, providing you resist! For snacks have cheese or a few nuts. Drink loads of water.
Have a good browse round the forums and see if you can pick up any tips. Come back with any questions.