In its natural state, the Metropolis Ensemble is a chamber orchestra that specializes in new music, mostly by young composers. But for “Love Letters to Haiti,” its Sunday evening concert at Le Poisson Rouge, Metropolis was really an umbrella group that brought together instrumental soloists, chamber ensembles, singers and a few composers for a benefit performance for Partners in Health, a nonprofit organization that provides medical care in Haiti (and elsewhere), and is currently working with victims of the Haitian earthquake.