RNDC command to dump stats

We have a request, from a resolver operator, to be able to issue an rndc command to dump a file of statistics in some format that can be imported and processed by Prometheus. I think the use case is, the node might be in a remote pop with limited connectivity and setting up on-going streaming may be difficult for some reason, or perhaps there isn't the access for that.

entered on behalf of cmosher@quad9.net