Original: It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis.
Simplified: Metis was found in 1979 by Stephen P. Synnott in images taken by the Voyager 1 probe. In 1983 it was named after the mythological Metis, a Titaness who was the first wife of Zeus (the Greek equivalent of Jupiter).