Feature Request: Metric Math support

[arunderwood]

Now that GetMetricData calls are supported, have you given thought to supporting Metric Math? I ask because certain AWS account information can only be retrieved via metric math.

For instance, if I want to export the used percentage of my KMS:DescribeKey quota, I need to call the function SERVICE_QUOTA() on the KMS:DescribeKey metric. To illustrate this, here is a description of a CloudWatch Dashboard that displays the metric:

{
    "metrics": [
        [ { "expression": "100 * m1 / SERVICE_QUOTA(m1)", "label": "Utilization %", "id": "e1", "region": "us-east-1" } ],
        [ "AWS/Usage", "CallCount", "Resource", "DescribeKey", "Service", "KMS", "Type", "API", "Class", "None", { "id": "m1", "visible": false } ]
    ],
    "period": 1,
    "units": "None",
    "stat": "Sum",
    "view": "timeSeries",
    "stacked": false,
    "title": "DescribeKey request rate - (Utilization %)",
    "region": "us-east-1"
}