Zubin Mehta is contemplating retirement as music director of the New York Philharmonic at the end of the 1990-91 season, The New York Times reported Wednesday. Mehta said that during his current yearlong sabbatical he has enjoyed being away from New York City and the grind of about 90 concerts a season. "The way it stands now, I'm leaving. But my contract has four years to go. If they come to me in two years' time with an offer, I will think about it then," Mehta said. When his contract expires at the end of the 1990-91 season, Mehta will have been the Philharmonic's music director longer than any other conductor.