Creates the specified index. A newly created index's initial state is CREATING. On completion of the returned google.longrunning.Operation, the state will be READY. If the index already exists, the call will return an ALREADY_EXISTS status. During index creation, the process could result in an error, in which case the index will move to the ERROR state. The process can be recovered by fixing the data that caused the error, removing the index with delete, then re-creating the index with create. Indexes with a single property cannot be created.