public void close() {
    if (closed) {
      throw new IllegalStateException(
          String.format("Attempted to close Operator %s more than once.", getOpImplId()));
    }
    handleClose();
    closed = true;
  }
Returns a registry which allows registering arbitrary system-clock timer with K-typed key.
The user-defined function in the operator spec needs to implement {@link ScheduledFunction#onCallback(Object, long)}
for timer notifications.
@param <K> key type for the timer.
@return an instance of {@link Scheduler}
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImpl.java