Merge SystemPublicMetrics in MetricRegistry when using dropwizard metrics.

[celiovasconcelos]

Spring actuator is awesome.
It would be nice to see system-metrics in MetricRegistry.
Devs are being forced to workaround like this.

[sshcherbakov]

+1

[wilkinsona]

This has been superseded by the planned move to Micrometer-based metrics (#9970)