The Amazon Resource Number (ARN) of the custom metric. For example, arn:<i>aws-partition</i>:iot:<i>region</i>:<i>accountId</i>:custommetric/<i>metricName</i>
arn:<i>aws-partition</i>:iot:<i>region</i>:<i>accountId</i>:custommetric/<i>metricName</i>