code: func (c *tokenSource) Token() (*oauth2.Token, error) {
  	return retrieveToken(c.ctx, c.conf, url.Values{
  		"grant_type": {"client_credentials"},
  		"scope":      internal.CondVal(strings.Join(c.conf.Scopes, " ")),
  	})
  }
docs: // Token refreshes the token by using a new client credentials request.
  // tokens received this way do not include a refresh token