default boolean isComplete() {
        return whenComplete().isDone();
    }
Returns {@code true} if this stream is complete, either successfully or exceptionally,
including cancellation and abortion.

<p>A {@link StreamMessage} is <em>complete</em> (or 'fully consumed') when:
<ul>
  <li>the {@link Subscriber} consumes all elements and {@link Subscriber#onComplete()} is invoked,</li>
  <li>an error occurred and {@link Subscriber#onError(Throwable)} is invoked,</li>
  <li>the {@link Subscription} has been cancelled or</li>
  <li>{@link #abort()} has been requested.</li>
</ul>
core/src/main/java/com/linecorp/armeria/common/stream/StreamMessage.java