Yes, it is generally okay to put a metal spoon in a fridge. Metal spoons, such as stainless steel or silverware, are typically safe to be stored in the refrigerator without causing any harm to the spoon or the food inside. In fact, many people store metal utensils in the fridge for convenience or to keep them cool for serving chilled dishes or beverages.
However, if you plan to store highly acidic or corrosive foods in direct contact with metal utensils for an extended period, it's best to use non-reactive materials like glass or plastic to avoid any potential chemical reactions or flavor changes. For regular storage of foods, including placing a metal spoon in the refrigerator temporarily, there should be no issues.