How to Create grpc api.pb.go?

Hi, all:

I was trying to create a new api.pb.go with api.proto.

``` bash
protoc --go_out=./ api.proto
```

can generate a new api.pb.go file, but in this file APIClient is missing( the formor api.pb.go do have APIClient!).

what the cause of this problem, am I using the wrong proto file?
