Photo: Spencer Platt/Getty Images Alvin Bragg does not like to, well, brag about his record as Manhattan District Attorney, but the numbers don’t lie . “The ultimate metric is that shootings are down during my tenure, down 70 percent in Manhattan. That’s the ultimate output,” he told me. Murders are down 52 percent. “I think the four-years-plus body of work speaks for itself.” The stats are part of New York’s contribution to the national debate over whether so-called progressive prosecutors