Add support to region Metric from CloudWatch Dashboard

[gc23]

The metric class is missing the region parameter. This is required for metrics around lambda@edge which are deployed in different locations.

The following is an example of the graph widget produced by the AWS console.

{
    "view": "timeSeries",
    "region": "eu-west-1",
    "stacked": false,
    "metrics": [
        [ "AWS/Lambda", "Invocations", "FunctionName", "us-east-1.FunctionName", "Resource", "us-east-1.FunctionName:1", { "yAxis": "left", "period": 300, "stat": "Sum" } ],
        [ "...", { "yAxis": "left", "period": 300, "stat": "Sum", "region": "eu-west-2" } ],
        [ "...", { "yAxis": "left", "period": 300, "stat": "Sum", "region": "eu-central-1" } ]
    ],
    "period": 300
}