For sitar, earliest I can think of is Metallica’s 'Wherever I may roam". Banjo might be Pride & Glory’s “Losing my mind”, or maybe something of Zakk’s earlier work?
Seems these instruments have become kinda common in the genre (or at least not ‘wtf was that’).
The Beatles used the sitar quite a bit in the sixties.
Here ya go, Paint It Black, 1966, waaaay before Metallica, and without a question rock music.
I don’t think the Beatles would be considered metal though. 
For some reason I had rock in my head and wondered why the OP only went back as far as Metallica.
…and I don’t think the Rolling Stones would be considered the Beatles, either. 
Not sure about the OP; checking my archives…
Yeah, I know. I knew the Beatles used the sitar quite a bit, but then I remembered Paint It Black and was trying to come up with the youtube link and the date and get it in during the edit window.
< smacks self >
I need to give up the internets.
Led Zeppelin used both. Do they count as metal?
Oh, that’s a good one. It’s not a band I would call “metal” from today’s perspective (I’d just put them into “hard rock”), but they were considered a heavy metal band once upon a time.