Instrument Scheduled methods for observability

This commit enhances the `ScheduledAnnotationBeanPostProcessor` to
instrument `@Scheduled` methods declared on beans. This will create
`"tasks.scheduled.execution"` observations for each execution of a
scheduled method. This supports both blocking and reactive variants.

By default, observations are no-ops; developers must configure the
current `ObservationRegistry` on the `ScheduledTaskRegistrar` by using a
`SchedulingConfigurer`.

Closes gh-29883

framework-docs/src/main/java/org/springframework/docs/integration/observability/tasksscheduled/ObservationSchedulingConfigurer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.integration.observability.tasksscheduled;
+
+
+import io.micrometer.observation.ObservationRegistry;
+
+import org.springframework.scheduling.annotation.SchedulingConfigurer;
+import org.springframework.scheduling.config.ScheduledTaskRegistrar;
+
+public class ObservationSchedulingConfigurer implements SchedulingConfigurer {
+
+	private final ObservationRegistry observationRegistry;
+
+	public ObservationSchedulingConfigurer(ObservationRegistry observationRegistry) {
+		this.observationRegistry = observationRegistry;
+	}
+
+	@Override
+	public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
+		taskRegistrar.setObservationRegistry(this.observationRegistry);
+	}
+
+}