To start with, don't eat the glass. Me, I'd throw it away. I guess you could go to Kroger and tell them - they MIGHT give you a free cake, or maybe a coupon or something.
^^^
LIKE
I think it is best if you take the cake and the piece of glass back to Kroger to show them what you found. Obviously glass does not belong in cakes. Kroger will want to make sure that none of their other cakes have glass pieces in them similar to what you found.
Because no one suffered an injury from eating the glass, however, you have no reason to sue. I agree with Zigner that Kroger might give you a coupon or a replacement cake but I don't think it is reasonable to expect much more than that.