Some additional information:
Light Metal is useless on Metagross because it weighs 550 kg, or 275 kg after Light Metal. However, some weight-based moves, like Low Kick or Grass Knot (nobody uses Knot on Metagross but still), deal maximum power at 200 kg, meaning they will still be 120 Base Power. As for Heat Crash and Heavy Slam, those actually get stronger the lighter the target is, so to minimize damage against Heat Crash, Metagross would actually prefer not to have Light Metal. As for the last weight-based move, Sky Drop, it fails if the target weighs over 200 kg, so no matter what ability Metagross has, Sky Drop will fail.
Just go for Clear Body, it blocks Intimidate and Sticky Web along with situational stat drops and is much more useful on a game to game basis.
Source: https://bulbapedia.bulbagarden.net/wiki/Category:Moves_affected_by_weight, along with the pages for the moves.