transport/http: add http handler (#937)

* add http handler

* fix request error

examples/http/handler/main.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"context"
+	"log"
+
+	"github.com/go-kratos/kratos/v2"
+	"github.com/go-kratos/kratos/v2/transport/http"
+	"github.com/gorilla/mux"
+)
+
+// User is a user model.
+type User struct {
+	ID   string `json:"id"`
+	Name string `json:"name"`
+}
+
+// UserService is a user service.
+type UserService struct{ users map[string]*User }
+
+// Get get a user from memory.
+func (u *UserService) Get(ctx context.Context, user *User) (*User, error) {
+	return u.users[user.ID], nil
+}
+
+// Add add a user to memory.
+func (u *UserService) Add(ctx context.Context, user *User) (*User, error) {
+	u.users[user.ID] = user
+	return user, nil
+}
+
+func main() {
+	us := &UserService{
+		users: make(map[string]*User),
+	}
+	router := mux.NewRouter()
+	router.Handle("/users", http.NewHandler(us.Add)).Methods("POST")
+	router.Handle("/users/{id}", http.NewHandler(us.Get)).Methods("GET")
+
+	httpSrv := http.NewServer(http.Address(":8000"))
+	httpSrv.HandlePrefix("/", router)
+
+	app := kratos.New(
+		kratos.Name("handler"),
+		kratos.Server(
+			httpSrv,
+		),
+	)
+	if err := app.Run(); err != nil {
+		log.Println(err)
+	}
+}