Peter Gelb’s first season as director of the Metropolitan Opera was coasting along at a breathless pace in late February, as the sold-out run of Tan Dun’s The First Emperor was bumping up against pre-production buzz over two additional spring premieres, each generating huge box office runs: Gluck’s Orpheus and Eurydice and Puccini’s Il Trittico. The last thing that the Gelb team expected to confront on February 28 was the bombshell from across Lincoln Center Plaza that New York City Opera, the perennial underdog to the Met, had named Gérard Mortier as the successor to retiring executive director Paul Kellogg, effective in 2009.