Matt Donnelly Senior Film WriterJennifer Lopez has always been obsessed with authenticity.It’s there in her music, like the 2014 single “Same Girl,” the radio sensation “I’m Real” with Ja Rule and her signature bop “Jenny From the Block.” In the latter, a girl from the Bronx makes the solemn promise: “No matter where I go, I know where I came from.” It’s also been doled out in her film roles, many of them rags-to-riches fairytales that emphasize both her street smarts and supreme glamour, like “Maid in Manhattan,” “Second Act” and, most recently, “Marry Me.”So the prospect of her all-access documentary – Netflix’s “Halftime,” which opened this year’s Tribeca Film Festival and debuts on the streamer today – not only makes perfect sense for J. Lo-the-brand but also suggests her larger obligation to the truth as she enters the fourth decade of a tremendous career in show business.