enhance log for hello world SayHello (#2538)
enhance log for hello world SayHello (#2538)

This PR adds a log to `SayHello`. This makes server logic more clear.
grpc/grpc-go,thinkerou/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,menghanl/grpc-go,stanley-cheung/grpc-go,grpc/grpc-go,thinkerou/grpc-go,dfawley/grpc-go,dfawley/grpc-go