! akka-cluster: metrics collector improvements - resolve #16121

there seems to be no consistent way of differentiating config-key vs type-name in ```reference.conf```; how about if we introduce new convention? :
```
config-key === `akka.cluster.metrics`
type-name  === [[akka.cluster.Metrics]]
```
for people who read scala all day long ```[[...]]``` would probably be self evident