Doesn't mean the rest of the world is like that. Metalbending is literally the heart of Zaofu's culture, as it is with the RCPD (being founded/run by the first metalbender).
It's not uncommon I bet, but I wouldn't assume most earthbenders are metalbenders, either. We've seen extremely potent professional benders like Bolin that can't pick it up, and he's the top 99th percentile of earthbenders at least.
Same with lightning bending. I don't get why people assumed it was common in LoK just because a few factory workers and mentee of a gangster boss (Mako) could do it, when powerful firebenders like Korra, P'Li, Zuko and such apparently can't.