OutgoingMessageEnvelope createMessageEnvelope(SystemStreamPartition ssp, int index) {
    return new OutgoingMessageEnvelope(ssp.getSystemStream(), String.valueOf(index), value);
  }
Naive create message implementation that uses the same random string for each of the message.
If a system producer wants to test with a specific type of messages, It needs to override this method.
samza-tools/src/main/java/org/apache/samza/tools/benchmark/SystemProducerBench.java