I stumbled upon this by accident and this is hardly metal or even rock, but...

Crimean -born Petr Ivanovich Isakov, a self-taught guitarist who was in 1913 even invited to the Winter Palace in Saint Petersburg to teach the children of Emperor Nicholas II to play the guitar, recorded this track in April 1913 (so that's 110 years ago).

It is his interpretation of a Russian folk song called "Oh, My Garden" ("Ах ты, сад"), which was originally released on "Amour Gramophone Record" in 1913-1914, which is interesting here. From 0:20 to 0:40 (approx.) there is a finger-picked (?) "riff" which, because of the technology constraints in those days, actually sounds now as it was performed on an electric guitar, and as such is the earliest example of something that sounds vaguely reminding an actual riff.

You can disagree with me, that's fine, but check it out by yourself

https://www.russian-records.com/details ... 263&lng=en

Direct Mp3 -link:

https://www.russian-records.com/data/me ... t-2458.mp3

The original realease has been on sale on eBay, and looks like this: https://www.ebay.com/itm/175108831183

This is actually pretty cool. I don't know that the riff sounds that explicitly 'metal' per se (especially from about 0:34 on where it gets a little more... major/upbeat/however one with more musical theory knowledge would describe it), but it would certainly be interesting to hear someone translate it into more pure metal and use it as inspiration for a track.
It sounds kind of double layered, too.