The "metal gods" are robots that have risen up and have come to destroy us all. It's a common theme in science fiction, but expressed more viscerally here, as Rob Halford sings:
Ripping men apart
Halford was reading a lot of sci-fi at the time.
Specific inspiration for the lyric came from the 1977 album cover for Queen's News Of The World, which shows a giant robot reaping destruction. The H. G. Wells novel The War Of The Worlds was also an influence.
"Metal God" became Rob Halford's nickname - he even trademarked the phrase in 2009. Halford earned the title by championing heavy metal for decades; Judas Priest came of age in the late '70s when punk was all the rage and metal was looked down upon.
And don't worry, it hasn't gone to his head: Halford comes from a working class background and has a keen sense of humor. He doesn't wear a crown or require fans to bow before him.
Halford wrote this song with Judas Priest guitarists K.K. Downing and Glenn Tipton. The band recorded it, along with the rest of the tracks on the British Steel album, at the Tittenhurst Park estate in England, which was owned by Ringo Starr, who bought it from John Lennon and Yoko Ono.
Some of Ringo's cutlery can be heard on the song. They didn't have handy access to sound effects, so they made their own, shaking a tray of knives and forks to create the sound of the metal gods descending on humanity.