docs: add reactor context propagation example

[wetted]

closes #10219

I believe we agreed in the meeting that a kotlin example is not necessary (nor appropriate).

[dstepanov]

I would make a note before the context is got that it was modified in some way. Otherwise, it doesn't make sense to do.

[wetted]

I would make a note before the context is got that it was modified in some way. Otherwise, it doesn't make sense to do.

Sorry, I'm not sure I understand what you mean "that it was modified". (this is all new to me). Can you clarify for me further, please.

[dstepanov]

@Get('/hello')
        Mono<String> hello() {
            // The propagated context is modified ( PropagatedContext.get().add / remove etc)
            PropagatedContext propagatedContext = PropagatedContext.get() // <1>
            return Mono.just('Hello, World')
                .contextWrite(ctx -> ReactorPropagation.addPropagatedContext(ctx, propagatedContext)) // <2>
        }

[wetted]

// The propagated context is modified ( PropagatedContext.get().add / remove etc)

I changed the adoc comment. See if that's OK.

[dstepanov]

Looks fine; the other option would be to add some propagation context element to make it make more clear. Please take a look @sdelamo if it's understandable.

[wetted]

@dstepanov I updated the example to add an element. See if that's OK. Thanks for the feedback.

[sonarqubecloud]

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[sdelamo]

@dstepanov should we do this for Kotlin as well?

[dstepanov]

For Kotlin we have KotlinCoroutinePropagation

[sdelamo]

@dstepanov do you think we should merge it in the current status?

[dstepanov]

Looks good