code: public static byte[] encodeBitString(int padBits, byte[] value)
      {
          checkArgument(padBits >= 0 && padBits < 8, "Invalid pad bits");
  
          byte[] lengthEncoded = encodeLength(value.length + 1);
          ByteArrayDataOutput out = ByteStreams.newDataOutput(2 + lengthEncoded.length + value.length);
          out.write(BIT_STRING_TAG);
          out.write(lengthEncoded);
          out.write(padBits);
          out.write(value);
          return out.toByteArray();
      }
docs: Encodes a bit string padded with the specified number of bits.
  The encoding is a byte containing the padBits followed by the value bytes.