Security observations are not setting their parent osbervation

[bclozel]

Original issue: spring-projects/spring-boot#33495

Spring Boot auto-configures the reactive ServerHttpObservationFilter ahead of the Spring Security WebFilter; the former creates an Observation for the entire handling of the web request. It looks like Spring Securityis starting an observation in ObservationReactiveAuthorizationManager, without looking into the reactive context if there is an existing one. In this case, the web observation should be set as the parent of the security one in order to have the expected resulting trace.

A contributor has tested these changes and reported that this is fixing the problem.