code: func NewClient(meta *metadata.Client, acc string) *http.Client {
  	return &http.Client{
  		Transport: newRoundTripper(meta, acc),
  	}
  }
docs: // NewClient returns an *http.Client which sets a GCE VM metadata token in the
  // Header of an *http.Request during Do. The audience for the token is the
  // *http.Request.URL.Host.