AWS RDS IO metric

[brendanheywood]

This metric cannot be collected directly, and this metric may ultimately live in local_aws but tracking it here for now.

This might need to call an aws api, or it may need to be run elsewhere and a metric pushed back in via a new generic WS api for saving a metric.

The metric may also be mixed in with other data eg if two apps share an rds instance. So need a very simply ratio to attempt to proportion the metric.