void invokeOnSubscribe() {
            if (invokedOnSubscribe) {
                return;
            }
            invokedOnSubscribe = true;

            if (downstreamExecutor.inEventLoop()) {
                invokeOnSubscribe0();
            } else {
                downstreamExecutor.execute(this::invokeOnSubscribe0);
            }
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java