CEDAR RAPIDS — Gov. Terry Branstad, Iowa and the nation’s longest-serving governor, has resigned and has been sworn in as United States ambassador to the People’s Republic of China.It has been a great honor to serve,” Branstad said as he ended his 22-plus years as governor at a Statehouse ceremony this morning.“There is no better job in the world than being the governor of the state that you love,” he said, “but sometimes we are called to serve in ways we had never imagined.”Branstad was sworn in as ambassador by federal Judge Steve Colloton in front of an audience of about 85 people, including his wife, Chris, their children and five of the Branstad grandchildren.