Under the K Bridge Park, the patch of green that turned the base of the Kosciuszko Bridge into one of North Brooklyn’s hottest concert venues, has a problem most cool venues don’t. It is a public park, and it gets no public money to operate. So the nonprofit that runs it, the North Brooklyn Parks Alliance, keeps the plants watered and the seating intact by throwing dance parties with Peggy Gou and charging you a fee to fund the grass.