MicrometerObservationListener stops Observation twice

[chemicL]

Since the introduction of ObservationValidator in Micrometer, reactor-core-micrometer can produce errors on cancelled Monos that already delivered a value.

Originally reported in micrometer-metrics/micrometer#5802 by @mathiaskinzler.

Expected Behavior

No error logs should happen.

Actual Behavior

io.micrometer.observation.tck.InvalidObservationException: Invalid stop: Observation 'reactor.observation' has already been stopped

Steps to Reproduce

	@Test
	void monoCancelAfterNext() {
		TestObservationRegistry observationRegistry = TestObservationRegistry.create();
		Mono<Integer> mono = Mono.just(1).tap(Micrometer.observation(observationRegistry));
		StepVerifier.create(Flux.from(mono).take(1).single())
		            .expectNext(1)
		            .expectComplete()
		            .verify();
	}

Possible Solution

Analyze and fix the state machine in MicrometerObservationListener.

Your Environment

• Reactor version(s) used: 3.7.3-SNAPSHOT
• Other relevant libraries versions (eg. netty, ...): io.micrometer:micrometer-bom 1.14.3

[chemicL]

@mathiaskinzler the issue should be resolved. If you get a chance to experiment with the 3.7.3-SNAPSHOT, any feedback is appreciated.

[mathiaskinzler]

@chemicL We have now updated to spring boot starter 3.4.3 and things work like a charm. Thanks for your support.

[chemicL]

@mathiaskinzler glad to hear it! Thanks a lot for the update :)