I'm going to have a spine fusion surgery using a metal plate in my back. What are the chances that my body will reject the metal plate?

Rare. The metals and plastics used in surgery are well tolerated. Some can cause allergy, but happily this is rare.
Very low. The large majority of modern spinal instrumentation is made of titanium. Allergy to titanium is exceedingly rare.
Minimal. Most studies reports that immune response to spinal hardware is less than 0.1%. Titanium is mainly used in orthopedic implants today & is very inert. If your surgery include usage of hardware talk to your surgeon. As far as your concern about the hardware, an immune reaction should be the least of your concern.
Unlikely. The metal used in spinal implants is titanium. It is MRI compatible and rarely causes any type of problem such as host response.