Prior to this commit, Spring Boot would auto-configure a customizer that
instruments `RestTemplate` through a `RestTemplateBuilder`. This would
install a request interceptor that instrumented client exchanges for
producing metrics.
As of spring-projects/spring-framework#28341, the instrumentation is
done at the `RestTemplate` level directly using the `Observation` API.
The `Tag` (now `KeyValue`) extraction, observation name and
instrumentation behavior now lives in the Spring Framework project.
This commit updates the auto-configuration to switch from Boot-specific
Metrics instrumentation to a generic Observation instrumentation.
As a migration path, some configuration properties are deprecated in
favor of the new `management.observations.*` namespace.
Closes gh-32484
eac50a8f0c