Clean up SolrInfoBean / SolrMetricProducer API

For historical reasons both SolrInfoBean and SolrMetricProducer contain methods and constants / enums that deal with handling and reporting of metrics. In almost all cases implementations of SolrInfoBean also implement SolrMetricProducer.

I propose to refactor this API so that SolrInfoBean simply extends SolrMetricProducer. This will reduce the API surface and eliminate multiple rote methods that subclasses must now implement.

This is an incompatible API change so it's applicable only to version 9.0.