Always get all RDS metrics.
Keep list of RDS metrics in code, not in configuration file.
Remove "name" from the configuration.
Remove "aws_namespace" from the configuration, always use "AWS/RDS".
Remove "aws_dimensions" from the configuration, always use "DBInstanceIdentifier".
Remove "aws_statistics" from the configuration, always use "Average".
Add a list of a region plus database instance name. It will be generated by pmm-managed.