Append dimensions ssm managed instance id support

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

Start at the ec2tagger handling for the `aws:InstanceId` append dimension and trace how instance metadata is resolved. Investigate the SSM registration details available to on-premise managed instances, then verify that the append_dimensions configuration resolves the managed instance ID without breaking EC2 behavior.

Written by the indexing model from the issue text.