See org.apache.lucene.util.IOUtils.spins. It currently only works for linux and is used by ConcurrentMergeScheduler to set defaults for maxThreadCount and maxMergeCount.
We should expose this as a metric for solr.data.home and each core's data dir. One thing to note is that the CMS overrides the value detected by the spins method using lucene.cms.override_spins system property. This property is supposed to be for tests but if it is set then the metrics API should also take that into account.