If there’s a Dive Bar Hall of Fame, surely the venerable Charlie’s Kitchen belongs in it. With its tattered vinyl booths, a killer jukebox, and “cheeseburger specials,” Charlie’s is a still-thriving throwback to Harvard Square’s scruffier days. It’s also a refreshingly egalitarian watering hole that draws neighborhood cops and shopkeepers, tattooed punk rockers, and, as the unofficial canteen of Harvard Kennedy School, some U.S. presidents. For almost five decades, legendary waitress Helen Metros, 84, has been happy to welcome them all.