We used the term heavy metal for a few years before Sabbath and the metal bands came out.
There was no official class of rock called metal, it was just what we called that heavy
oversaturated riff-oriented stoner rock of the late sixties early seventies. Hendrix, Tull, Yes, Zeppelin, all metal.
Seems to me the first real popular metallic-sounding tune was "Satisfaction" by the Stones. That was
the first super-heavy riff out there, really groundbreaking, proto-metal for sure. But probably not the first. The heavy electric guitar riffing and improv seemed to start somewhere in the surf-rock of the early sixties, "Miserlou" by Dick Dale comes to mind. A top contender for the "invention" of heavy rock, and a big influence on the likes of Pete Townshend, who was certainly one of the first axe murderers, was Link Wray, probably the first real distortion crunch-hammer, real early on.
Pooneil Corners really pushed the envelope, but by then lots of bands were banging their guitars pretty hard. Metal certainly owes more to the British Invasion than the San Francisco sound. Which, in my opinion, is very much to the credit of the acid-rockers.