I want to create a Cloudwatch alarm for Memory Utilisation across an ASG.

This is the SQL query I want to replicate in Terraform:

SELECT AVG(mem_used_percent)
FROM SCHEMA(CWAgent, AutoScalingGroupName,ImageId,InstanceId,InstanceType)
WHERE AutoScalingGroupName = 'my-asg-name'

This is my current Terraform code:

resource "aws_cloudwatch_metric_alarm" "memory_utilization_high" {
  alarm_name          = "HighMemoryUtilizationAcrossASG"
  comparison_operator = "GreaterThanOrEqualToThreshold"
  evaluation_periods  = 3
  threshold           = 80
  alarm_actions       = [aws_sns_topic.asg_alerts.arn]
  treat_missing_data  = "breaching"

  metric_query {
    id          = "e1"
    expression  = "AVG([m1])"
    label       = "Average Memory Utilisation across ASG"
    return_data = "true"
  }

  metric_query {
    id = "m1"
    metric {
      metric_name = "mem_used_percent"
      namespace   = "CWAgent"
      period      = 300
      stat        = "Average"
      unit        = "Percent"

      dimensions = {
        AutoScalingGroupName = var.asg_name
        InstanceId           = ""
        InstanceType         = ""
        ImageId              = ""
      }
    }
  }
}

I get this Terraform error:

error ValidationError: 3 validation errors detected: Value '' at 'metrics.2.member.metricStat.metric.dimensions.2.member.value' failed to satisfy constraint: Member must have length greater than or equal to 1; Value '' at 'metrics.2.member.metricStat.metric.dimensions.3.member.value' failed to satisfy constraint: Member must have length greater than or equal to 1; Value '' at 'metrics.2.member.metricStat.metric.dimensions.4.member.value' failed to satisfy constraint: Member must have length greater than or equal to 1

I have tried removing these dimensions:

InstanceId = ""
InstanceType = ""
ImageId = ""

and Terraform is happy with it but Cloudwatch doesn’t show any metrics. The SQL query at the top though does what I need.

Anybody has any idea how to fix this? Thanks!