You should not attempt to destroy metal credit cards. If you request a replacement card or decide to close your account, the credit card company will likely send you a slip in which to return your old metal card when it mails you a new one. Or, they'll just ask you to mail the old metal card to a particular address, without providing an envelope or any such supporting materials. The issuer will then dispose of the card safely.
Procedures vary by credit card company. So you should call your card's customer service department to see how it recommends dealing with an unused metal credit card.
You definitely don't want to take a knife to a metal credit card or try to put such a card through a shredder. That's a recipe for a broken tool and perhaps a trip to the hospital. With that in mind, the bottom line is: Let credit card companies destroy metal credit cards so you don't have to.