cloudwatch: Add resource_arns template query function

Implements feature request #8207

docs/sources/features/datasources/cloudwatch.md

@@ -74,6 +74,12 @@ Here is a minimal policy example:
                 "ec2:DescribeRegions"
             ],
             "Resource": "*"
+        },
+        {
+            "Sid": "AllowReadingResourcesForTags",
+            "Effect" : "Allow",      
+            "Action" : "tag:GetResources",      
+            "Resource" : "*"      
         }
     ]
 }
@@ -128,6 +134,7 @@ Name | Description
 *dimension_values(region, namespace, metric, dimension_key, [filters])* | Returns a list of dimension values matching the specified `region`, `namespace`, `metric`, `dimension_key` or you can use dimension `filters` to get more specific result as well.
 *ebs_volume_ids(region, instance_id)* | Returns a list of volume ids matching the specified `region`, `instance_id`.
 *ec2_instance_attribute(region, attribute_name, filters)* | Returns a list of attributes matching the specified `region`, `attribute_name`, `filters`.
+*resource_arns(region, resource_type, tags)* | Returns a list of ARNs matching the specified `region`, `resource_type` and `tags`.
 
 For details about the metrics CloudWatch provides, please refer to the [CloudWatch documentation](https://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/CW_Support_For_AWS.html).
 
@@ -143,6 +150,8 @@ Query | Service
 *dimension_values(us-east-1,AWS/RDS,CPUUtilization,DBInstanceIdentifier)* | RDS
 *dimension_values(us-east-1,AWS/S3,BucketSizeBytes,BucketName)* | S3
 *dimension_values(us-east-1,CWAgent,disk_used_percent,device,{"InstanceId":"$instance_id"})* | CloudWatch Agent
+*resource_arns(eu-west-1,elasticloadbalancing:loadbalancer,{"elasticbeanstalk:environment-name":["myApp-dev","myApp-prod"]})* | ELB
+*resource_arns(eu-west-1,ec2:instance,{"elasticbeanstalk:environment-name":["myApp-dev","myApp-prod"]})* | EC2
 
 ## ec2_instance_attribute examples