appengine.go
func WithContext(parent context.Context, req *http.Request) context.Context {
	return internal.WithContext(parent, req)
}
func WithContext(parent context.Context, req *http.Request) context.Context {
	return internal.WithContext(parent, req)
}
// WithContext returns a copy of the parent context
// and associates it with an in-flight HTTP request.
// This function is cheap.