Add SystemMetricsProducer to spring autoconfiguration

[mairovichaa]

SystemMetricsProducer isn't part of Spring autoconfiguration.

I suggest to add a possibility to turn on its autoconfiguration by means of application's properties.

For example,

# application.yml
...
management:
    metrics:
        mf:
            collect:
                system: true
...

In this case
org.springframework.boot.actuate.autoconfigure.metrics.JvmMetricsAutoConfiguration
and
org.springframework.boot.actuate.autoconfigure.metrics.SystemMetricsAutoConfiguration
should be excluded from autoconfiguration as they collect similar metrics.