cloudwatch plugin support for metrics without dimension

[cjander]

The cloudwatch plugin with it's methods query() and query_one() do always require a defined dimension and also does validate it's content. But there are metrics in cloudwatch that do not have any dimension. so these metrics cannot be monitored using zmon. although there is support for "*" and "NOT_SET" for metrics names in the plugin, this still requires a dimension.

In our example we need to monitor SNS SMS costs, the cloudwatch metric for these is created automatically by SNS and it has no dimension:

[cjander]

another example is SES Reputation.ComplaintRate, Reputation.BounceRate, Send, Delivery, Complaint and Bounce statistics