BEIJING/SHANGHAI, July 5 (Reuters) - China's export controls on metals used in making semiconductors are "just a start", an influential trade policy adviser said on Wednesday, as it ramps up a tech fight with the U.S. days before U.S. treasury secretary Janet Yellen visits Beijing. Shares in some Chinese metals companies rallied for a second session, with investors betting that higher prices on gallium and germanium, which Beijing's export restrictions target, could boost revenues. Germanium is used in high-speed computer chips, plastics, and in military applications such as night-vision devices as well as satellite imagery sensors. Gallium is used in...