Thanks for your question.
It is a pleasure to know that your kidneys returned to its normal functioning. But it is a frustrating situation to gain weight with insulin. Anyway, metformin is associated with weight loss. You can go on a trial period of 1 or 2 months with metformin to check whether it is beneficial. But you should stop it if it is found causing any trouble to kidneys. It is because health of kidneys is more of concern than weight loss. Byetta is ordered orally for diabetes patients. But before starting, make sure that you are using only one at a time. Either Byetta or metformin. Otherwise, you may not be able to find which one is causing which effect.
Stay healthy and fit. Good luck.