Adding custom ObservationHandler does not preserve the default ObservationHandlers

Spring Boot version

3.0.4

Reproducible example

https://github.com/violetagg/netty-observation-repro

Problem description

I would like to use Reactor Netty tracing capabilities in Spring Boot application.
For this purpose I need to reuse the ObservationRegistry created by Spring Boot and to add ObservationHandlers provided by Reactor Netty.

Observations:

1. If I only reuse the ObservationRegistry without adding the ObservationHandlers provided by Reactor Netty - the tracing information is correctly visualised but the span tags are duplicated and one and the same Timer object is created every time.

2. If I reuse the ObservationRegistry and add ObservationHandlers provided by Reactor Netty - the tracing information is NOT correctly visualised.

3. If I reuse the ObservationRegistry, add ObservationHandlers provided by Reactor Netty and in addition add all default ObservationHandlers provided by Spring Boot - the tracing information is correctly visualised, the span tags are not duplicated and one and the same Timer object is created only once. Reactor Netty tracing capabilities are working as expected.

Desired solution

Spring Boot should preserve the default ObservationHandlers when custom ObservationHandlers are added to the configuration.

Start with Spring Boot's ObservationRegistry configuration and the custom ObservationHandler setup described in the issue, then reproduce the behavior with the linked netty-observation-repro example. Trace how adding Reactor Netty handlers affects Spring Boot's default handlers; done means custom handlers coexist with the defaults and the expected tracing, span tags, and timer behavior are preserved.