code: @Override
      public DescribeSessionsResult describeSessions(DescribeSessionsRequest request) {
          request = beforeClientExecution(request);
          return executeDescribeSessions(request);
      }
docs: <p>
  Retrieves a list that describes the active streaming sessions for a specified stack and fleet. If a value for
  <code>UserId</code> is provided for the stack and fleet, only streaming sessions for that user are described. If
  an authentication type is not provided, the default is to authenticate users using a streaming URL.
  </p>
  
  @param describeSessionsRequest
  @return Result of the DescribeSessions operation returned by the service.
  @throws InvalidParameterCombinationException
  Indicates an incorrect combination of parameters, or a missing parameter.
  @sample AmazonAppStream.DescribeSessions
  @see <a href="http://docs.aws.amazon.com/goto/WebAPI/appstream-2016-12-01/DescribeSessions" target="_top">AWS API
  Documentation</a>