King Bumi definitely did not bend metal (remember, he was captured by the Fire Nation and put in a metal crypt; of course, he might have been playing down his capabilities to save his people, as he told Aang, but I don't think he could bend metal; otherwise, wouldn't he have used it when he fought Aang?).
As far as the audience knows, Toph is the only metal bender.
The chains used to bring down the Generals by the Dai Li were metal; obviously, the earth-bending Generals couldn't have been captured with the rock manacles (not chains) used by the Dai Li on the other characters. Good point by "obsessedchic1" in the description of a device used to launch the chains!