Coldplay humbly opened the concert, chiming its way through its mopey anthems with Chris Martin bounding all over the stage as he sang; the band also unveiled a brand-new song, “Amazing Day,” which was ideal for getting a crowd slowly swaying. Mr. Sheeran, the guitar-strumming songwriter, used electronic loops to construct full-scale rock arrangements in real time, complete with drumming (tapped on his guitar) and backup chorales (his own voice), building promises of love up to galloping, syncopated peaks.