title: Creating a CloudWatch alarm based on a search expression
body: <p>I'm attempting to the do the following: </p>
  
  <p>I have a DynamoDB global table which publishes the ReplicationLatency metric. I want to create an alarm on the aggregate of the ReplicationLatency metric published for each region. </p>
  
  <p>The DDB table replicas exist in us-east-1, us-west-2 and us-west-1. In defining the CW alarm for each receiving region, I was under the assumption that I could a search expression. For example, here's the expression I see in the CloudWatch console.</p>
  
  <pre><code>SEARCH('{AWS/DynamoDB,ReceivingRegion,TableName} MetricName="ReplicationLatency"', 'Average', 300)
  </code></pre>
  
  <p>I'd like to create a metric math alarm which is the avg of the metrics of the above search result. I was attempting to create a metric math expression of the format: </p>
  
  <pre><code>AVG(METRICS())
  </code></pre>
  
  <p>I then get the following error - The expression for an alarm must include at least one metric. Has anybody attempted to create an alarm from a search expression before? If yes, could you shed some light on how it can be done? </p>
  
  <p>The only other way I can think of solving this problem is to enumerate/add the ReplicationLatency metric for each receiving region and then create a metric math expression out of that. That's seems to completely defeat the purpose of having a search expression and creating an alarm from all those metrics. </p>