Re: Who says a Tele cannot rock?
The earliest metal-sounding band I ever heard was Mountain.
The first band I ever heard labelled 'Heavy Metal' was Led Zeppelin, but this was in Rolling Stone magazine and after they had already been playing for almost 5-10 years - so I found that perplexing to retroactively call them something when they already had been known as a heavy blues band with some acoustic folk and variety mixed in, up to that point.
In my mind, the muted-string chugging of chords is what I first associated as metal or heavy metal. Early examples were Scorpions and Sabbath. But I have no idea who was really first.
It didn't just suddenly happen where one day someone invented metal, though. Completely non-metal/rock acts had hints of that sound coming over the years - like Mick Ronson on Suffragette City with David Bowie had that heavy distorted Marshall chugging. Before that Velvet Underground had a taste of that heavy chugging on some songs as well.