Metropolitan Opera soprano Roberta Peters announced Thursday she will become the first US singer to perform in China in 30 years. Her programs will offer music of Mozart and Schubert, along with American songs and a folk tune she is learning in Chinese. She will also teach classes at Peking Conservatory and several universities. Miss Peters's 15-day visit in Peking, Shanghai, and Canton will be the third US musical event in China since normalization of relations. Last year the Boston Symphony Orchestra and violinist Isaac Stern staged separate tours.