Document broker-side socket-server-metrics

There are a bunch of broker JMX metrics in the "socket-server-metrics" space that are not documented on kafka.apache.org/documentation

• MBean: kafka.server:type=socket-server-metrics,listener=<listenerName>,networkProcessor=<processorIndex>
  • From KIP-188: https://cwiki.apache.org/confluence/display/KAFKA/KIP-188+-+Add+new+metrics+to+support+health+checks
•  kafka.server:type=socket-server-metrics,name=connection-accept-rate,listener={listenerName}
  • From KIP-612: https://cwiki.apache.org/confluence/display/KAFKA/KIP-612%3A+Ability+to+Limit+Connection+Creation+Rate+on+Brokers

It would be helpful to get all the socket-server-metrics documented