Make ObservationRegistry non final in BatchMetrics

This makes it possible to assign a pre-configured
ObservationRegistry (by Spring Boot for instance)
in BatchMetrics

spring-batch-core/src/main/java/org/springframework/batch/core/observability/BatchMetrics.java

@@ -58,7 +58,7 @@ public final class BatchMetrics {
 	 * Global {@link ObservationRegistry}. A {@link DefaultMeterObservationHandler} is
 	 * attached to create a {@link Timer} for every finished {@link Observation}.
 	 */
-	public static final ObservationRegistry observationRegistry;
+	public static ObservationRegistry observationRegistry;
 
 	static {
 		observationRegistry = ObservationRegistry.create();