timestamp method
Wraps each item emitted by the source Stream in a Timestamped object that includes the emitted item and the time when the item was emitted.
Example
Stream.fromIterable([1])
   .timestamp()
   .listen((i) => print(i)); // prints 'TimeStamp{timestamp: XXX, value: 1}';
Implementation
Stream<Timestamped<T>> timestamp() =>
    TimestampStreamTransformer<T>().bind(this);