Adapt UserController example to current ServerRequestObservationContext

See gh-30013

framework-docs/src/main/java/org/springframework/docs/integration/observability/httpserver/reactive/UserController.java

@@ -28,7 +28,7 @@ public class UserController {
 	@ExceptionHandler(MissingUserException.class)
 	ResponseEntity<Void> handleMissingUser(ServerWebExchange exchange, MissingUserException exception) {
 		// We want to record this exception with the observation
-		ServerRequestObservationContext.findCurrent(exchange)
+		ServerRequestObservationContext.findCurrent(exchange.getAttributes())
 				.ifPresent(context -> context.setError(exception));
 		return ResponseEntity.notFound().build();
 	}