$result = $this->client->getMetricStatistics(array(
        'Namespace'  => 'My.Namespace',
        'MetricName' => 'my-metric',
        'StartTime'  => strtotime('-1 days'),
        'EndTime'    => strtotime('now'),
        'Period'     => 300,
        'Statistics' => array('Average'),
));

array (size=7)
  0 =>
    array (size=3)
      'Timestamp' => string '2014-04-16T10:53:00Z' (length=20)
      'Unit' => string 'Count' (length=5)
      'Average' => string '9.998594711316002' (length=17)
  1 =>
    array (size=3)
      'Timestamp' => string '2014-04-16T11:43:00Z' (length=20)
      'Unit' => string 'Count' (length=5)
      'Average' => string '0.7908148450858722' (length=18)
  2 =>
    array (size=3)
      'Timestamp' => string '2014-04-16T11:08:00Z' (length=20)
      'Unit' => string 'Count' (length=5)
     'Average' => string '5.402251656252796' (length=17)
  3 =>
    array (size=3)
      'Timestamp' => string '2014-04-16T11:03:00Z' (length=20)
      'Unit' => string 'Count' (length=5)
      'Average' => string '8.958888493053081' (length=17)