@Override
  public String toString() {
    String wndKey = "";
    if (!(key instanceof Void) && key != null) {
      wndKey = String.format("%s:", key.toString());
    }
    return String.format("%s%s", wndKey, paneId);
  }
A (key,paneId) tuple uniquely identifies an emission from a window. For instance, in case of keyed-tumbling time windows,
the key is provided by the keyExtractor function, and the paneId is the start of the time window boundary. In case
of session windows, the key is provided by the keyExtractor function, and the paneId is the time at which the earliest
message in the window arrived.
samza-api/src/main/java/org/apache/samza/operators/windows/WindowKey.java