Metropolitan Opera Centennial Gala. Paramount. $79.95. The sprawling birthday concert of the Metropolitan Opera--call it a vocal super-circus--began early on Oct. 22, 1983, and ended well after midnight. The video memento runs well short of four hours. Although expendable contributions by the chorus and ballet have been retained, along with endless curtain calls, much has been lost and, thanks to awkward editing, much has been confused. Not all the performances that remain are genuinely stellar--opera idols aren't what they used to be. Nevertheless, a historic occasion like this deserves better documentation.