The Metformin will do nothing to combat the sugar from your cakes and jam or anything else you eat. It's not insulin it does not remove any sugar from your blood, it's main function is to limit the amount of stored glucose released by your liver. On average it seems that Metformin will lower your hba1c by a massive 1.5% , if you're one of the unfortunates who suffer the more unpleasant side effects, the benefits hardly seem worth it.
We can't give medical advice but I can tell you what I did. Metformin side effects made my life unbearable so I chucked them all in the bin and cut a few more carbs from my diet. Medication is supposed to make our lives better not worse, your nurse can not force you to take any drugs, no one can. Medical professionals can advise us about our treatment but the final decision is yours.
Ps
A quick Google search will provide you with lots of low carb cake and dessert recipes, you can even make jam with strawberries and chia seeds I believe.