SPOILER WARNING: This story discusses plot details for “ The Devil Wears Prada 2 ,” in theaters now. Last fall, Lady Gaga ’s fans worked themselves into a frenzy when they spotted her in Milan amid her sold-out Mayhem World Tour. Her presence was conspicuous since she was in between shows in London and Stockholm — plus the long-awaited sequel to “The Devil Wears Prada,” starring Meryl Streep, Anne Hathaway, Emily Blunt and Stanley Tucci, was filming in the Italian city for fashion week. It didn’t take long for Gaga’s devotees to put together — or for Variety to report — that Mother Monster would indeed make an appearance in the movie opposite Miranda Priestly. But further details of her involvement were kept under wraps. Now, with “The Devil Wears Prada 2” in theaters, the filmmakers can reveal how the A-list cameo came together — all thanks to a personal call by Meryl Streep. Streep and Gaga met at “SNL 50” last February and immediately hit it off.…