While other metals have shined in 2016, copper has struggled to gain traction and last week erased its gains for the year. Demand in China, the world’s biggest user, is slowing just as Goldman Sachs Group Inc. predicts a “supply storm” will hit the market and drag prices even lower. Stockpiles of the metal are ballooning, further pointing to a demand slowdown. Inventories monitored by the London Metal Exchange have jumped to a...