title: Fetch AWS Cloudwatch metrics for all RDS instance using Wildcard
body: <p>For some alerting and monitoring purpose, I am trying to fetch RDS cloudwatch metrics.
  There are around ~50 RDS instances and would like to fetch metrics for all of them in one API call(I am using boto3). This is my code :</p>
  
  <pre><code>response = cloudwatch_client.get_metric_data(
      MetricDataQueries=[
          {
              'Id': 'fetching_data_for_something',
              'MetricStat': {
                  'Metric': {
                      'Namespace': 'AWS/RDS',
                      'MetricName': 'FreeStorageSpace',
                      'Dimensions': [
                          {
                              'Name': 'DBInstanceIdentifier',
                              'Value': '*'
                          },
                      ]
                  },
                  'Period': 300,
                  'Stat': 'Average'
              },
              'ReturnData': True
          },
      ],
      StartTime=datetime(2019, 6, 11,13,0,0),
      EndTime=datetime(2019, 6, 11,13,20,00),
      ScanBy='TimestampDescending',
      MaxDatapoints=123
  ) 
  </code></pre>
  
  <p>But this is returning an empty result, but when I am searching for a particular DB instance, it is returning the result. This is working :</p>
  
  <pre><code>'Dimensions': [
  {
      'Name': 'DBInstanceIdentifier',
      'Value': 'name_of_db'
  },
  ]
  </code></pre>
  
  <p>But, this isn't : </p>
  
  <pre><code>'Dimensions': [
  {
      'Name': 'DBInstanceIdentifier',
      'Value': '*'
  },
  ]
  </code></pre>
  
  <p>Is there any way to accomplish this? To get metrics for all DBs at once?</p>