Discussed this with markmiller.
SOLR-8869 allows us to not return field cache entries from the SolrFieldCacheMBean. It would be nice to be a little more flexible about this; the reason SOLR-8869 was useful was that the field cache printing was expensive due to periodic monitoring calls to /jmx. But turning off the field cache entry printing via SOLR-8869 also turns it off from the web ui. It can be useful for users and admins to be able to view the cache entries there, non-periodically, without paying a performance penalty due to /jmx calls.
The proposal here is to allow SolrInfoMBeans to define statistics to be returned specificially for jmx purposes and to implement a specific instance of this for the SolrFieldCacheMBean.