I've always taken metformin up to the day before, as I was told, and never had a problem. They did need to give me a glucose drip once during surgery when it was postponed from morning to afternoon, which is very likely why they have asked you to have a light breakfast.
I do know that some contrast dyes used for MRI scans can cause problems for people taking metformin, and you do have to stop taking metformin a few days before if they are planning to use a contrast dye but I've never had a scan where they've needed to do that.
I'm sure you'll be fine and I hope everything goes well for you.