Custom global dimensions

Hi! Thank you for the software.
I'd like to ask what was the reason to forbid specifying custom global dimensions? I would like to add a global custom dimension `environment name: ` for ALL metrics from cloudwatch agent for example. Yes, I can add it manually for most of agent inputs, but I can't do this for statsd and collectd, for example.

I'm talking about this part:

```
append_dimensions – Optional. Adds Amazon EC2 metric dimensions to all metrics collected by the agent. The only supported key-value pairs are shown in the following list. Any other key-value pairs are ignored.

"ImageID":"${aws:ImageId}" sets the instance's AMI ID as the value of the ImageID dimension.
"InstanceId":"${aws:InstanceId}" sets the instance's instance ID as the value of the InstanceID dimension.
"InstanceType":"${aws:InstanceType}" sets the instance's instance type as the value of the InstanceType dimension.
"AutoScalingGroupName":"${aws:AutoScalingGroupName}" sets the instance's Auto Scaling group name as the value of the AutoScalingGroupName dimension.
```

May be you are planning to add this feature or you can suggest any workaround.
Thanks!

The request centers on append_dimensions and the StatsD and collectd inputs; start by tracing how those configuration paths add dimensions to collected metrics. Done means a user-defined global dimension such as an environment name is accepted and appears on metrics from all mentioned inputs, including StatsD and collectd.

Written by the indexing model from the issue text.