Sandy Pearlman, reviewing the Stones’s Got Live if You Want It! in issue #8 of Crawdaddy! (March 1967):
On this album the Stones go metal. Technology is in the saddle — as an ideal and as a method. A mechanically hysterical audience is matched to a mechanically hysterical sound. Side two of the album is a metal side. Most mechanical. It has the historic “Last Time,” one of the Stones’ first big metal songs but sounding pretty tame in this company, a very metallic “Time is On My Side,” without the mellow yellow organ of the first try. A metal “I’m Alright”; and a moderately metal “Satisfaction” with metal mitigation supplied by Billy Wyman’s newly super-miked bass, which sounds as if San Francisco in August and the Airplane and Jack Cassady might have had something to do with it. It also has a significant merger of the metallic and the morbid…
Is this the earliest use of “metal,” as applied specifically to rock? I personally always think of “metal” as following on the heels of “heavy metal” (much in the way that “rock” followed on the heels of “rock and roll,” and much in the same way that “Led Zeppelin” begat “Zeppelin” which in turn begat “Zep”), and yet, according to Wikipedia, “the first documented use of the phrase [heavy metal] to describe a type of rock music identified to date appears in a [May 1968 Rolling Stone] review [of Electric Flag] by Barry Gifford.”* In other words, Pearlman leapfrogged past the still-impending heavy metal sound to prop up what he heard in ’67 as simply metal (and with his persistent use of the term “mechanical,” he could just as well be writing about Voivod or someone) — a pretty neat trick, when you think about it.
* “Nobody who’s been listening to Mike Bloomfield — either talking or playing — in the last few years could have expected this. This is the new soul music, the synthesis of white blues and heavy metal rock.”
RockCritics.com 
how about this Axis: Bold as Love (btw, could this have been what Chandler was thinking of in that BBC thing?) review in Rolling Stone:
By Jim Miller
April 6, 1968
Jimi Hendrix sounds like a junk heap (Ben Calder crushed monolithic mobiles bulldozed), very heavy and metallic loud. Rock’s first burlesque dancer, superman in drag, his music is schizophrenic. Axis: Bold as Love is the refinement of white noise into psychedelia, and (like Cream) it is not a timid happening; in the vortex of this apocalyptic transcendence stands Hendrix, beating off on his guitar and defiantly proclaiming “if the mountains fell in the sea, let it be, it ain’t me.” Such cocky pop philosophy shall not go unrewarded.
That’s a good one, too. What’s interesting, probably, about all these early examples, is that the writers all seem to be using the word “metal” (and its derivatives) as an actual adjective, to get at how the music sounds. This is all before it was codified into a genre that everyone understood. (I think “grunge” has similar beginnings… was often used as a word by critics to describe certain types of loud, dirty guitar rock.)
“In the Saddle” hmmm…
Perhaps he was just testing their mettle…