Major Description
To configure SolrSlf4jReporter we must configure a logger in the log4j2.xml file and then the solr.xml file must be able to reference it.
It won't be super obvious to users on how to do this so we should show an example in the ref guide
With log4j.xml we could do it like https://github.com/vthacker/solr-solutions/blob/master/docs/solr_metrics_logger.md but not we moved over to log4j2.xml
I have a working example for log4j2.xml and I'll post a patch in the next few hours