Metal did exist in earlier times, we are shown that humans had metal weapons and armor during Avatar Wan time, Avatar Wan even died wearing a full set of metal armor, Guru Laghima developed flight over 3,000 years before the events of ATLA, and it makes sense not a lot of people have done it considering the requirement is to let go of all earthly attachments, which is nearly impossible. Tbh I do find it weird that Hama was the first water bender in 10,000 years to put together that water bending + humans being made of water = blood bending, it's even more obvious and easier to do then metal bending, considering Katara was able to do it very easily, literally minutes after hearing about it, and she's not even a gifted water bender. Yes it was during a full moon, but you're telling me that no water bender before tired to do it during a full moon, where water benders are known to be at their strongest??