Well-met does show up in some dictionaries:
adjective Archaic. (used as a salutation or part of a salutation). ...Origin: 1580-90.
Hail-fellow-well-met can be traced back to the same time:
1580s, from a familiar greeting
as @HaL's answer explains further.
As for "Well met," it's used as a greeting in writing at least through the early 1900s - L. Frank Baum uses it, for example. (In this case it's used as a greeting to a new person - as today we would use nice to meet you. I think later examples may have that meaning more than earlier ones, but I am not sure.)
"Well met, Stranger!" cried the Patchwork Girl with a whoop of laughter. "You are quite the funniest individual I have seen in all my travels."
Readings from Lectures to working men, Rev. Arthur Mursell, ~1867, indicates that the phrase meant something like good to see you, but perhaps was not one all speakers would use:
Going back further, Shakespeare uses "well met" liberally, for example:
—Well met, honest gentlemen.
—By my troth, well met; Come, sit, sit, and a song.
—You are well met once again.
—And so are you.
as a "kind of salutation" (clip from a Shakespeare dictionary that cross-references where words and phrases occur):