Use Resolved ObservationRegistry in DataSourceObservationListener bean

[ttddyy]

Based on #54, when background entity manager creation is enabled, the EntityManager is created in a separate thread. As part of EntityManager creation, it accesses the database to obtain database metadata. The JDBC access invokes a call to DataSourceObservationListener, which also triggers the ObjectProvider<ObservationRegistry> to lazily retrieve the ObservationRegistry bean for DataSourceObservationListener. Since this call is made in a separate thread, it causes a deadlock in the BeanFactory. (Details described in spring-projects/spring-framework#34247)

Originally, the lazy resolution of ObservationRegistry was introduced to avoid circular references with Micrometer registry, binders, and filters during Spring Boot startup. This behavior was addressed at some point (possibly in this commit).

To address this issue, change the DataSourceObservationListener bean definition to directly use the resolved ObservationRegistry bean instead of relying on ObjectProvider<ObservationRegistry>.

To preserve the existing ObjectProvider-based lazy resolution behavior, it could add a property to toggle this behavior.