Add a spins metric for all directory paths

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.