Unwrap metric_data

`format_param/1` was not working correctly for nested fields. Each field
was being prefixed with MetricData.member but the inner fields were not
getting their specific prefixes. This caused dimensions to not be sent
correctly.

This fixes #1

lib/ex_aws/cloudwatch.ex

@@ -626,7 +626,9 @@ defmodule ExAws.Cloudwatch do
   end
 
   defp format_param({:metric_data, metric_data}) do
-    metric_data |> format(prefix: "MetricData.member")
+    metric_data
+    |> Enum.flat_map(&format_param/1)
+    |> ExAws.Utils.format(prefix: "MetricData.member")
   end
 
   defp format_param({:ok_actions, ok_actions}) do