SpringBoot Fix Observation lifecycle issues in ObservationAutoConfigurationTests

This PR fixes Observation API lifecycle issues in ObservationAutoConfigurationTests: - Observations should not started twice - Observations should be stopped

so that we can rely on tests that use the API as it should be used and they can also provide a good example. These issues we discovered while I was working on https://github.com/spring-projects/spring-boot/pull/32399

Comment From: wilkinsona

Thank you, @jonatan-ivanov.