While Metroid was released a little before Zelda II was officially released (August 6th 1986 vs January 14th 1987), Zelda II was already in development when Metroid was begun, though it had none of the elements of being a Zelda game; in fact, the Zelda-specific elements actually only came into existance at the last phase of development, when they were they were trying to craft a story and bring it all together (which is why it's such a radical departure from the first game in so many ways, though it introduced many elements to the series that have remained consistant throughout the series since; most notably, the Triforce of Courage being in Link's possession).
Furthermore, the entire concept of adding RPG mechanics to a sidescroller of any sort actually originated with Zelda II; there were no others before it that did so, though there were plenty in the shmup genre especially that did have powerups of one sort or another, such as Gradius or Knightmare; other precursors of later genres, like Thexder (which would later inspire such classics as Contra and Metal Slug) didn't even have those, though those inspired by it usually had upgrades of one sort or another.