Fixes: #2901
* Rework an observation for Rabbit Binder
The observation propagation doesn't work in multi-binder configuration
* Remove `ObservationAutoConfiguration` since it is not visible in case of multi-binder configuration
* Instead move `setObservationEnabled` flag setting to the `RabbitMessageChannelBinder`
* Add `io.micrometer.observation.ObservationRegistry` into `shared.beans` to make it visible for binder-specific application context
* Add `RabbitMultiBinderObservationTests` integration test where Rabbit Binder is in a multi-binder environment
As a side effect, this fixes an observation propagation for the Kafka binder as well in the multi-binder environment.
Its configuration is OK, but an `ObservationRegistry` must make it visible for the binder-specific application context.
See the mentioned `shared. beans`
Related to https://github.com/spring-cloud/spring-cloud-stream/issues/2901
Also see https://github.com/spring-cloud/spring-cloud-stream/issues/2902 for possible evolution
* Update the copyright of the affected classes
3e24d331db