most people need about 75 - 80 g of protein a day. This is estimated based on your body weight x 0.8 - 1 g of protein / kg body weight.
Protein foods are not all protein. For example 1 egg has about 6 g of protein.
1 Oz of meat also about 6 g of protein.
Protein foods are eggs, nuts, cheese, as well as meats.
If you are really having problem you could try things like smoothie made with protein powder.
Many people find that the tummy problems with metformin are short lived and they can return to their normal eating after a week or two once their body gets used to the medication.