There was a piece of metal in the food I bought at a shop, and I had put it in my mouth. I want to claim for medical expenses if it makes me sick.

As soon as possible, take the food and the metal fragment (foreign matter) to the shop where you bought the food and have a talk. However, if you are not showing any signs of health issues, and it is uncertain if any health hazard will arise from the incident, it will be difficult to claim medical expenses from the shop for potential damages.