Just one year after a video of the band Unlocking the Truth playing in Times Square went viral, they were signed to a $1.8 million recording contract. They played at SXSW and Coachella. They opened for Metallica and Guns N’ Roses. It was a meteoric rise for three black 13-year-olds from Brooklyn. A filmmaker named Luke Meyer followed them with his camera through it all, and the result is a documentary called “Breaking a Monster.” The subtitle might have been ‘be careful what you wish for’.