Adding custom variable in cloudwatch agent config

Question

I have the following config that I generate as part of user data for my EC2 instance

{
        "agent": {
                        "run_as_user": "root"
        },
        "logs": {
                "logs_collected": {
                        "files": {
                        "collect_list": [
                                {
                                "file_path": "/var/log/hapee-2.0/lb-access*",
                                "log_group_name" : <insert-region-here>
                                "log_stream_name": "haproxy-{instance_id}"
                                }
                        ]
                }
                }
        },
       "metrics": {
                "metrics_collected": {
                    "net": {
                    "measurement": [
                        "net_packets_recv",
                    "net_packets_sent"
                    ],
                    "metrics_collection_interval": 60
                },
                "mem": {
                        "measurement": [
                                "used_percent"
                        ]
                }
        }

        }
} 

As the log_group_name I want to be able to append the region that the instance is running in. I understand that the region is part of the instance meta data but can I use it in the cloudwatch config?