For the 30th Bridge School Benefit Concert, a capacity crowd at the Shoreline Amphitheatre in Mountain View received metal and water in the form of Metallica and Roger Waters, both “shed” headliners in their own right. Singer-songwriter/multi-instrumentalist Nils Lofgren touchingly serenaded the the students from the Bridge School, which serves children with severe disabilities, and families who sat at the back of the stage. Augmented with a string quartet, Cage the Elephant sounded at ease in an “unplugged” setting. Backed by his Family Band (including son Lukas Nelson on Saturday), country music star Willie Nelson kick-started his set with “Whiskey River” and included favorites such as “Beer for My Horses,” “Mama Don’t Let Your Babies Grow Up to Be Cowboys” and a heartfelt version of “Always on My Mind.” The only set not to feature a guest appearance, adopt an atypical instrumentation or incorporate a cover song, singer-songwriter/acoustic guitarist Dave Matthews and acoustic guitar wizard Tim Reynolds were tight and energized. Armed with an acoustic guitar and a sense of wonder, Roger Waters opened by singing “Wish You Were Here” with Wolfe, Laessig, My Morning Jacket’s Jim James and guitarist G.E.