I don't know if this is more of a glitch or a cheat, but it makes it easy to get your metal count raised to the max almost instantaneously.

Start by getting one of the Life Tank Rare Metals in either the Optic Sunflower or Dark Mantis' stage.

Buy a bunch of items until your metal count is below the cost of a Life Charge (this is less than 50 without the metal discount item). Once you're below that amount, buy a life charge. Your metal count will go to 0. Now enter any stage, and pick up any metal. Your metal count will start rapidly increasing. Now pause the game, and your metal count will be completely maxed out.