Most CO2 lasers used by hobbyists, maker spaces, and small businesses aren't intended for use with metal. An alternative to actually engraving the metal is to use a marking chemical that bonds to the metal when the laser is applied.There are several options that work to varying degrees, at a range of costs, and intended for various metals.Dry Moly is an inexpensive lubricant that works well as a marking material on stainless steel. View on eBayCerMark is a leading commercial product designed for this purpose. There are various types of CerMark that work on a variety of metals. View on eBay
Now here's a weird one, apparently mustard can be used to engrave on metal too!