Uh, and why would you not insert food-safe metal (my favorite example being a skewer out of a turkey lacing kit) into a cake?
And as to food-safe metal touching the cake or frosting, well, cake pans are usually made of metal, so you have metal touching the cake before it even makes it into the oven. And informal cakes are frequently served in-pan, so you end up, unless you're really neat with the frosting, with metal touching the frosting.
The key question is whether or not the metal is food-safe. And personally, I wouldn't trust anything not specifically marketed as being for food contact, regardless of whether it's metal, plastic, wood, ceramic (remember, serving food on most "collectors' plates" is not good for the food, or the plate!), stone, or glass, unless I knew the exact composition, and where it's been.