add grpc example (#1401)

use grpc helloworld example.

examples/grpc/grpc/server.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"log"
+	"net"
+
+	pb "github.com/gin-gonic/gin/examples/grpc/pb"
+	"golang.org/x/net/context"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/reflection"
+)
+
+// server is used to implement helloworld.GreeterServer.
+type server struct{}
+
+// SayHello implements helloworld.GreeterServer
+func (s *server) SayHello(ctx context.Context, in *pb.HelloRequest) (*pb.HelloReply, error) {
+	return &pb.HelloReply{Message: "Hello " + in.Name}, nil
+}
+
+func main() {
+	lis, err := net.Listen("tcp", ":50051")
+	if err != nil {
+		log.Fatalf("failed to listen: %v", err)
+	}
+	s := grpc.NewServer()
+	pb.RegisterGreeterServer(s, &server{})
+
+	// Register reflection service on gRPC server.
+	reflection.Register(s)
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}