Racked by unrelenting pain, Mexican painter Frida Kahlo wrote in her diary shortly before she died: “I joyfully await the exit — and hope never to return.” Yet return she does — if only briefly — on the Day of the Dead in “El Último Sueño de Frida y Diego” (“The Last Dream of Frida and Diego”), a Spanish-language opera receiving its Metropolitan Opera premiere this week. The opera, with libretto by playwright Nilo Cruz and music by Gabriela Lena Frank, imagines Kahlo’s spectral reunion three years after her death with Diego Rivera, the great Mexican muralist with whom she had a tempestuous romantic relationship. In a twist on the Orpheus legend, Rivera has grown weary of life without Kahlo and — on the holiday that honors the dead and welcomes the return of their spirits — he summons her from the underworld in the hope they may be eternally reunited.…