docs: function listLabels(auth) {
      var gmail = google.gmail('v1');
      gmail.users.labels.list({
          auth: auth,
          userId: 'me',
      }, function(err, response) {
          if (err) {
              console.log('The API returned an error: ' + err);
              return;
          }
          var labels = response.labels;
          if (labels.length === 0) {
              console.log('No labels found.');
          } else {
              console.log('Labels:');
              for (var i = 0; i < labels.length; i++) {
                  var label = labels[i];
                  console.log('- %s', label.name);
              }
          }
      });
  }