Never watched the original Avator series so I didn't know it is rare for an Earth bender to become a metal bender. Korra picked it up very easily it seemed.
Actually Toph invented metalbending when she was 12, there were no other metal benders in the OG series besides her.
I dunno if it's that rare now since she founded the metalbending police (and presumably taught all of them), but I don't think most benders would be able to learn it without training.
But I think some of the stuff that was rare in the OG series isn't as rare now. For example lightning bending used to be very rare (Zuko wasn't even able to learn it) but now you got nyggas in factories doing it just as a job.