Record errors thrown by custom handler in RestTemplate observations

Prior to this commit, the `RestTemplate` observation instrumentation
would only record `RestClientException` and `IOException` as errors in
the observation. Other types of errors can be thrown by custom
components, such as `ResponseErrorHandler` and in this case they aren't
recorded with the observation.
Also, the current instrumentation does not create any observation scope
around the execution. While this would have a limited benefit as no
application code is executed there, developers could set up custom
components (such as, again, `ResponseErrorHandler`) that could use
contextual logging with trace ids.

This commit ensures that all `Throwable` are recorded as errors with the
observations and that an observation `Scope` is created around the
execution of the client exchange.

Fixes gh-32060

spring-web/src/main/java/org/springframework/web/client/RestTemplate.java

@@ -880,7 +880,7 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 		Observation observation = ClientHttpObservationDocumentation.HTTP_CLIENT_EXCHANGES.observation(this.observationConvention,
 				DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, this.observationRegistry).start();
 		ClientHttpResponse response = null;
-		try {
+		try (Observation.Scope scope = observation.openScope()){
 			if (requestCallback != null) {
 				requestCallback.doWithRequest(request);
 			}
@@ -894,7 +894,7 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 			observation.error(exception);
 			throw exception;
 		}
-		catch (RestClientException exc) {
+		catch (Throwable exc) {
 			observation.error(exc);
 			throw exc;
 		}