protected void putMessageValue(String key, String value) {
    getMessageValues().put(key, value);
  }
@param key
          The key inside the messageMap, please only use human readable string (no JSON or such) - this allows
          easy mutation of the coordinator stream outside of Samza (scripts)
@param value
          The value corresponding to the key, should also be a simple string
samza-core/src/main/java/org/apache/samza/coordinator/stream/messages/CoordinatorStreamMessage.java