Append dimensions ssm managed instance id support

[ghost]

Is your feature request related to a problem? Please describe.
We use the following config snippet for our ec2 instances;

"metrics": {
  "append_dimensions": {
    "InstanceId": "${aws:InstanceId}"
  }
}

This doesn't work on our on premise instances, registered as ssm managed instances, as ec2tagger doesn't know how to look for the ssm registration details.

Describe the solution you'd like
Modify aws:InstanceId or add another value that allows us to resolve the managed instance id for on-premise installation.