[bugfix] ensure the generated data can be returned in time when in multi flux which have dependency

[flycloud-hz]

When dealing with multi-level Flux connections with dependencies, concurrent execution is required to ensure the generated data can be returned as quickly as possible

What's changed?

@PostMapping(value = "/a2a/server4", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<JSONRPCResponse> dispatchStream4(JSONRPCRequest request) {
    Sinks.Many<JSONRPCResponse> sink2 = Sinks.many().multicast().onBackpressureBuffer();
    Mono<Void> mono = Mono.fromRunnable(() -> {
        log.info("xxx fromRunnable mono-1");
        sink2.tryEmitNext(createResponse("mono-1"));
    }).then(Mono.delay(Duration.ofMillis(1000))).then(Mono.fromRunnable(() -> {
        log.info("xxx fromRunnable mono-2");
        sink2.tryEmitNext(createResponse("mono-2"));
    })).then(Mono.delay(Duration.ofMillis(1000))).then(Mono.fromRunnable(() -> {
        log.info("xxx fromRunnable mono-3");
        sink2.tryEmitNext(createResponse("mono-3"));
    })).then(Mono.delay(Duration.ofMillis(1000))).then(Mono.fromRunnable(() -> {
        log.info("xxx fromRunnable mono complete");
        sink2.tryEmitComplete();
    })).then();
    Flux<JSONRPCResponse> flux2 = sink2.asFlux().timeout(Duration.ofSeconds(10));

    // option-wrong:
    //        return mono.thenMany(flux2)
    //                .doOnNext(event -> log.info("xxx doOnNext {}", event))
    //                .map(event -> {
    //                    log.info("xxx map {}", event);
    //                    return event;
    //                });

    // option-right
    return Flux.merge(mono.thenMany(Mono.empty()), flux2).doOnNext(event -> log.info("xxx doOnNext {}", event))
            .map(event -> {
                log.info("xxx map {}", event);
                return event;
            });
}

The above code describes the returned two different results about the multiple flux which have dependency.

1. in "option-wrong" way

The data is returned in one time almost.

2. in "option-right" way

The data is returned in time.

The data which is emitted by sink should be returned as quickly as possible, so use flux.merge instead of thenMany.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Any Else Note

[flycloud-hz]

@tomsun28 Sorry to bother you, but could you please help review this?

[tomsun28]

@flycloud-hz Sorry for slow response, LGTM 👍