Mahler’s five-movement Symphony No. 7 has its share of shadows, especially in its two mysterious “night music” movements, but all is not gloom. The massively scored symphony—it includes cowbells, mandolin, and guitar—culminates in a wildly exuberant finale where snippets of operetta, a reference to the opening of Wagner’s Die Meistersinger, and a Turkish march joyously swirl together.