DefaultServerRequestObservationConvention is silently used when multiple convention beans are defined

[breun]

With Spring Boot 3.1.6, a Spring WebFlux application with Micrometer Observation would fail to start when multiple beans of type ServerRequestObservationConvention are defined, which I consider expected behavior. However, with Spring Boot 3.2.0 this behavior changed, and now you don't get a startup failure when multiple ServerRequestObservationConvention beans are defined, but silently DefaultServerRequestObservationConvention is used. I think this is unexpected and confusing behavior.

I've created https://github.com/breun/spring-boot-reactive-observation-convention-issue to illustrate this issue. See the tests on the spring-boot-3.1 and spring-boot-3.2 branches of this project.

[scottfrederick]

The reason that you're no longer getting the exception when multiple ServerRequestObservationConvention beans are created is that the WebFlux observation auto-configuration no longer uses beans of this type. See spring-projects/spring-boot#37344 for the background on this change. This is not currently mentioned in the 3.2 release notes, so we should add something there.

[bclozel]

We'll align the Framework behavior with the previous Spring Boot setup.