For the monitoring client the current doc examples are causing confusion as it generates interval = {} and gives no hint to the type or contents of interval. a startTime and endTime need to be provided. This PR updates the docstrings to be a bit more helpful. Is it possible to do this in the generator?

googleapis/nodejs-monitoring#176

From other bug, instead providing, for instance (in node):

   * // Use an interval of the last 60 seconds
   * const interval ={
   *     startTime: { seconds: Date.now() / 1000 - 60,},
   *     endTime: { seconds: Date.now() / 1000,},
   * }