default Optional<Integer> getBlockSize(BlockCacheKey key) {
    return Optional.empty();
  }
Returns an Optional containing the size of the block related to the passed key. If the block is
not in the cache, returned optional will be empty. Also, this method may not be overridden by
all implementing classes. In such cases, the returned Optional will be empty.
@param key for the block we want to check if it's already in the cache.
@return empty optional if this method is not supported, otherwise the returned optional
        contains the boolean value informing if the block is already cached.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCache.java