“The Plains Indians: Artists of the Earth and Sky,” one of the greatest exhibitions of American Indian art you may ever see, opened Monday at the Metropolitan Museum of Art. The show was originated by the Musée du Quai Branly in Paris and the Nelson-Atkins Museum of Art (where I saw it) in Kansas City, Mo., and organized by the inestimable Gaylord Torrence, the Nelson-Atkins’s curator in the field and also the orchestrator of its acclaimed permanent collection galleries.