If you keep a musician friend for over 50 years, as the experimental director Ping Chong has done with Meredith Monk, just maybe at your retirement celebration, that friend will sing you a song. And so on Wednesday night at the performance space Chelsea Factory, a luminous Monk sat down at a keyboard, reminisced about Chong when she first knew him — as a pony-tailed student in her dance class, wearing bell-bottom jeans — and played “Gotham Lullaby.”