Add missing ServiceLevelObjectiveBoundary hint

Closes gh-1538

Author: sdeleuze

spring-native-configuration/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/MetricsEndpointAutoConfigurationHints.java

@@ -29,6 +29,7 @@
 	@TypeHint(types = {
 		PropertiesConfigAdapter.class,
 		SimplePropertiesConfigAdapter.class,
+		org.springframework.boot.actuate.autoconfigure.metrics.ServiceLevelObjectiveBoundary.class,
 		org.springframework.boot.actuate.autoconfigure.metrics.ServiceLevelObjectiveBoundary[].class,
 	}),
 	@TypeHint(types = {