title: Show RDS Metrics for multiple instances
body: <p>I would like to pull RDS Cloudwatch Metrics using Boto for multiple databases at once. </p>
  
  <p>So far I have only been able to get metrics for only one instance at a time using an approach like this:</p>
  
  <pre><code>botoRDS = boto.connect_cloudwatch(aws_access_key_id=Key, aws_secret_access_key=OtherKey)
  
  instanceStats = botoRDS.get_metric_statistics(period=60, 
                  start_time=self.startTime,
                  end_time=self.endTime, 
                  namespace="AWS/RDS", 
                  metric_name='CPUUtilization', 
                  statistics=["Average"],
                  dimensions={'DBInstanceIdentifier':['DB1','DB2']})
  </code></pre>
  
  <p>This is what I get:</p>
  
  <pre><code>[
  {
      u'Timestamp': datetime.datetime(2034,1,14,21,2),
      u'Average': 45.1,
      u'Unit': u'Percent'
  }]
  </code></pre>
  
  <p>What I would like to be able to return is the average for both the database seperately. Something like this:</p>
  
  <pre><code>[
  {
      u'Timestamp': datetime.datetime(2034,1,14,21,2),
      u'DBInstanceID':'DB1',
      u'Average': 33.02,
      u'Unit': u'Percent'
  },
  {
     u'Timestamp': datetime.datetime(2034,1,14,21,2),
     u'DBInstanceID':'DB2',
      u'Average': 45.1,
      u'Unit': u'Percent'
  }
  </code></pre>
  
  <p>]</p>
  
  <p>Is it possible to form the dimension specified to get results like this. I would really like to not have to pull data for every DB.</p>