default Optional<Map<String, Long>> getRegionCachedInfo() {
    return Optional.empty();
  }
Returns an Optional containing a map of regions and the percentage of how much of it has been
cached so far.
@return empty optional if this method is not supported, otherwise the returned optional
        contains a map of current regions caching percentage.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCache.java