Code displaying an URL rather than the reponse of the request GCP with Go

I'm having hard times trying to display the result of a direction request in Google Cloud Platform. This is what I have tried:

package main

import (
    "fmt"
    "net/http"
    "context"
    "io/ioutil"
    "google.golang.org/appengine"
    "google.golang.org/appengine/urlfetch"
)

const directionAPIKey = "MyAppKey"
const directionURL = "https://maps.googleapis.com/maps/api/directions/json?origin=%s&destination=%s&mode=%s&key=%s"

func main() {
    http.HandleFunc("/", handler)
}

func handler(w http.ResponseWriter, r *http.Request) {
    ctx := appengine.NewContext(r)
    direction, err := fetchDirection(ctx, r.FormValue("origin"), r.FormValue("destination"), r.FormValue("mode"))
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    w.Header().Add("Content-Type", "application/json; charset=utf-8")
    w.Write(direction)
}

func fetchDirection(ctx context.Context, origin string, destination string, mode string) ([]byte, error) {
    client := urlfetch.Client(ctx)
    resp, err := client.Get(fmt.Sprintf(directionURL, origin, destination, mode, directionAPIKey))
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()
    return ioutil.ReadAll(resp.Body)
}

I got my app successfully deployed, however, the result of the above code is:

Get https://maps.googleapis.com/maps/api/directions/json?origin=&destination=&mode=&key=APIKey not an App Engine context

Rather than the result of the request. This is an image from the browser. How can I get the desired result and not only that URL? Thanks in advance.

I found that the issue "not an App Engine context" is returned when your AppEngine application has the runtime set to go111 and the application imports any of the "google.golang.org/appengine" packages but the application doesn’t call the “appengine.Main”

method func main() {
    http.HandleFunc("/", handler)
    appengine.Main()
}