def meta_keys
      zlist = ffi_delegate.meta_keys
      first_key = zlist.first
      return [] if first_key.null?
      keys = [first_key.read_string]
      while key = zlist.next
        break if key.null?
        keys << key.read_string
      end
      keys
    end
def meta_keys
      zlist = ffi_delegate.meta_keys
      first_key = zlist.first
      return [] if first_key.null?
      keys = [first_key.read_string]
      while key = zlist.next
        break if key.null?
        keys << key.read_string
      end
      keys
    end
Returns meta keys set.
 @return [Array<String>]