private byte[] fillRandomBytes(byte[] in) {
    new Random().nextBytes(in);
    return in;
  }
Fills random bytes into {@code in}.
@param in the byte array that needs to be filled with random bytes.
@return {@code in} filled with random bytes.
ambry-commons/src/test/java/com/github/ambry/commons/ByteBufferReadableStreamChannelTest.java