Add request to Observation Context at creation to enable filtering by the request

This issue was originally discussed in https://github.com/spring-projects/spring-boot/issues/34400.
The scenario is the following: if Spring Security is used and users want to ignore /actuator endpoints, there is no easy way to ignore Observations created by Spring Security because the details to make this decision are missing when the ObservationPredicate is tested.

One potential solution would be adding the request to the Context before the Observation is created, one example here: https://github.com/spring-projects/spring-security/blob/ac1d269e731275cef4ba2739cae6abe092a66234/web/src/main/java/org/springframework/security/web/ObservationFilterChainDecorator.java#L195-L202

Would be something like:

FilterChainObservationContext beforeContext = FilterChainObservationContext.before(request);
FilterChainObservationContext afterContext = FilterChainObservationContext.after(request);

Expected behavior: having extra details on the Observations created by Spring Security that allows the users to ignore these Observation based on the request.

There is a reproducer in the issue mentioned above.