spurious ERRORs due to race condition between SolrIndexSearcher metrics and IndexReader closing

I've seen situations in the wild where systems monitoring/polling metrics can trigger scary looking - but otherwise benign - ERRORs due to AlreadyClosedExceptions if/when the searcher/reader is in the process of being re-opened and the Gauge tries to call reader.numDocs(), etc...

We should tweak the metrics logic to just ignore these exceptions