With friend safaris, each slot has a chance of having a different Pokémon in it. There is a chance that the second slot in a Steel friend safari will be Metang.
So find someone with a Steel friend safari with Metang, ask for their friend code and register them to catch a Metang from their friend safari.
You can't actually catch Beldum, Metang or Metagross in X or Y, so the only way to get one is by getting one from a friend safari, or by migrating one from a previous game.
I hope I helped!