这里是golang代码，funcnewXXX返回一个接口(interface)，为什么不返回一个structtype_ABitOfEverythingServerstruct{vmap[string]*examples.ABitOfEverythingmsync.Mutex}typeABitOfEverythingServerinterface{examples.ABitOfEverythingServiceServer//interfaceexamples.StreamServiceServer//interface}funcnewABitOfEverythingServer()AB