you maybe only need to inform your insurance company, and have them note you are diabetic, then they can't use it against you if you come in trouble, but actually metformin is a very weak medication and I guess you are trying to control your condition and get your numbers down to normal, which is possible to most if they eat less than 100-150 grams of carbs in total daily...
I told my insurance just to be safe, then they may tell you that you only need to contact them again if your medication has been changed in future or/and if your condition has gone worse.. I pay the same as all others for travel insurance, my diabetes is under control and not really a health problem at the moment... you can do that too I am sure..