Integrate Reactor debugging features in WebFlux [SPR-17573]

[spring-projects-issues]

Rossen Stoyanchev opened SPR-17573 and commented

Reactor debugging features include:

1. Stack trace collection at assembly time, i.e. Hooks.onOperatorDebug(). Spring Boot enables this with devtools and it's also controlled via property spring.reactor.stacktrace-mode.enabled.
2. checkpoint operator with description string. This is of interest since it allows attaching information to exceptions in production without much overhead.
3. log operator for signals. Potentially of interest at TRACE log level to track emissions or and/or detect completion of processing stages.
4. tag operator for monitoring tools.

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Affects: 5.1.3