The Metropolitan Opera’s program at the end of this season includes Michael Mayer’s production of Verdi’s “Rigoletto,” set in a 1960s Las Vegas casino; the complete “Ring Cycle” — Richard Wagner’s four-part masterwork, directed by Robert Lepage; and “Dialogues des Carmélites” — Poulenc’s opera about an order of nuns whose faith is tested during the height of the French Revolution.