Guest List of the Masters Who Summer at the Met
LEAD: EVERY summer, while their owners spend a few months in the South of France or some similar locale, paintings by artists like Canaletto, Gauguin and Rembrandt take up residence on the walls of the Metropolitan Museum of Art. These seasonal guests are distinguishable from works in the permanent collection only by labels that describe them in small print as belonging to an anonymous collector, although it is impossible for the casual observer to know which of the paintings are on long-term loan and which are just passing through.