docs: private static byte[] buildFilterableHeader(byte[] contextID) {
          final byte[] header = new byte[Ints.BYTES + contextID.length];
          final byte[] length = Ints.toByteArray(contextID.length);
          System.arraycopy(length, 0, header, 0, length.length);
          System.arraycopy(contextID, 0, header, length.length, contextID.length);
          return header;
      }