I think that's a different aspect of metal that Judas Priest was central in developing. However, powerful, operatic sytle was already being developed within rock by acts like Led Zeppelin and then Queen. Heck, the idea of big, emotional vocals with sweeping music can be seen throigh soul from the 60s and 70s as well.
What I'm getting at is that the more epic and grandiose dismension of metal was a development upon something that was already quite refined within popular music (which isn't to say that metal hasn't done something special with it, as it obviously has). However, the bleak, heavy, nihilistic, and doomy dimension of metal really something new within popular music.