In 1969 there was no such thing as Metal except as a lyric in a Steppenwolf song. In 1969 all this genre-izing of band music didn't exist and it created one of most explosive and creative eras in music as bands as diverse as the Association, King Crimson, Jefferson Airplane, Jethro Tull and the Who were all just "rock" music. I don't think of Zep as metal or even proto-metal but within the historical context, J.P's use of layered guitars and J.B's drums definitely pushed rock in a heavier direction and were considered heavy by the standards of 1969. It also wasn't until the second album that the "heavy" really kicked in with songs like Heartbreaker and Whole Lotta Love though Dazed and Confused from LZ 1 certainly had a big sound.
I would posit that the genre we now call metal started with Black Sabbath which was what? 1970? I would also posit that The Who gave birth to arena rock and punk rock with the same windmill strum of a guitar.
Before someone gets after me for over simplifying. Nobody invents anything in music. Nothing comes from nothing; it always has an antecedent. It's like asking, did Beethoven invent symphonic music? No, but it certainly changed direction after him but before that Bach took the strict baroque forms of the melodious Telemann and gave us new ways to think of harmony which Handel then turned into the Sonata form which Mozart elevated to high art and then Beethoven came along and added about 30 members to the orchestra and some double basses and gave us dat-dat-dat DAH! and nothing was ever the same again. I don't know if Page and friends had quite the same impact on the music world; we'll have to wait another couple hundred years to sort that out but the fact that there's no musical instrument forum, guitar, bass or drums, where the names Zeppelin, Page, Bonham or John Paul Jones don't come up at least weekly after 47 years shows some lasting impact and influence.