How to do client streaming with Flux argument to HttpRequest

[RonBarkan]

It is required to "upload" a programmatically generated content. The content might be large an it is not desired to store the entire content in memory at any given time. I am trying to use a low-level HttpClient for this.
This is not documented here, here nor here, but I was able to find some hints (1, 2) that this might be allowed.

I have some code that sends JSON requests to some server and I've tweaked it to do the following:

   // var req = HttpRequest.POST(uri, jsonString);          // Original line (this works!)
   var req = HttpRequest.POST(uri, Mono.just(jsonString));  // Replacement (does not work)
    req.contentType(contentType)
        .accept(MediaType.APPLICATION_JSON)
        .bearerAuth(context.getAccessToken());

    return Mono.from(client.exchange(req, String.class));

When I run the code against a real server, I see that:

• POST header is sent and after a long deadline, I get 500 from the server (in the case of a long deadline), or timeout exception (in the case of the default client config)
• The Mono I introduced is not subscribed to by anything (I have added a .doOnSubscribe(...) that is never called)
• Tracing shows that the POST header is being sent with Transfer-Encoding: chunked, but then there's no more traces of the body being sent, so maybe it is never sent (tracing is done with <logger name="io.micronaut.http.client" level="TRACE"/>)
• I have used curl to send an identical message to the server, chunked encoded, and I get success

Because of all the magic and the abstractions, it is really not clear what's wrong. I expected Mono.just(jsonString) to work without issue.
From the documentation, it's unclear what is the relationship between Flux, Mono and content type.

Can you please help explain what might be wrong here, especially if the body is never sent to the server.
Note that the server and protocol cannot be changed. Mono.just() is used here only as a first step, to see that it works (it does not).
Using Micronaut v3.10.1

[RonBarkan]

@yawkat
@timyates
@graemerocher
Any suggestions?

[graemerocher]

Use jsonStream method

[graemerocher]

not sure if at this stage we support streaming the request body, maybe @yawkat knows

[yawkat]

publisher should work, not sure why it doesnt here.