protected void setType(String type) {
    this.keyArray[TYPE_INDEX] = type;
  }
Contains all fields for the message. This includes who sent the message,
the host, etc. It also includes a "values" map, which contains all values
associated with the key of the message. If set-config/job.name were used as
the type/key of the message, then values would contain
{"value":"my-job-name"}.
samza-core/src/main/java/org/apache/samza/coordinator/stream/messages/CoordinatorStreamMessage.java