The main reason Toph is the one to "invent" metal bending is because she's got tremor sense, which is a direct result of her being blind and learning from the moles. She was able to literally see where to push and pull the impurities.
What I want to know is how/when she was able to get lessons from them and how long it took, but considering she was also somehow able to become an underground fighting champion despite her helicopter parents I don't really care.
But that Guru Aang visits at the end of season 2 vaguely implies that other people did metal bending as well at some point, or at least that the concept was something that'd been looked into before.
Look, I can name a few instances in MY life where I tried to reach mutual understanding
and i can TELL you, always faster and easier to just kill em. Just is!