如何在terraform 0.11中将字符串转换为地图？

如何将字符串转换为地形图0.11.1

variable "custom_aws_tag" {
   default = "key1=value1,key2=value2,key3=value3"
} 

我希望以上述字符串格式从用户处获取键值对，并将其分配给AWS标记。Terraform的资源“aws_实例”接受地图格式的标签

resource "aws_instance" "app" {
   tags = "${merge(
     "${var.custom_aws_tag}",   <=== Needs to be interpolate to map before merging
     map(
       "app_name","acme",
       "env", "prod",
     )
   )}"
}

key1:value1
key2:value2
key3:value3
app_name:acme
env:prod