Nearly all the issuers of the metal cards provide a secure means of disposing them - at least for me, every time we get a new Chase card, the old ones are deactivated by Chase before we send them back.
I like to swipe a powerful magnet across any of the cards I have to wipe the strip, and then cut the chip in half along with cutting the card up in multiple pieces. Should anyone actually try and go through the effort of putting any card back together, they've all been deactivated by the issuer by that time so it would be really easy to see if any purchases are fraudulent.