Add ContextPropagatingTaskDecorator

Prior to this commit, `@Async` and `@EventListener` annotated methods
would lose the the logging and observation contexts whenever their
execution was scheduled on a different Thread.

The Context Propagation library supports this use case and can propagate
context values in ThreadLocals, Reactor Context and more.

This commit introduces a new `TaskDecorator` implementation that
leverages the Context Propagation library. When configured on a
`TaskExecutor`, this allows to properly propagate context value through
the execution of the task.

This implementation is completely optional and requires the
"io.micrometer:context-propagation" library on the classpath. Enabling
this feature must be done consciously and sometimes selectively, as
context propagation introduces some overhead.

Closes gh-31130

spring-core/src/test/java/org/springframework/core/task/support/ContextPropagatingTaskDecoratorTests.java

@@ -0,0 +1,100 @@
+/*
+ * 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.core.task.support;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import io.micrometer.context.ContextRegistry;
+import io.micrometer.context.ContextSnapshotFactory;
+import io.micrometer.context.ThreadLocalAccessor;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ContextPropagatingTaskDecorator}.
+ * @author Brian Clozel
+ */
+class ContextPropagatingTaskDecoratorTests {
+
+	@Test
+	void shouldPropagateContextInTaskExecution() throws Exception {
+		AtomicReference<String> actual = new AtomicReference<>("");
+		ContextRegistry registry = new ContextRegistry();
+		registry.registerThreadLocalAccessor(new TestThreadLocalAccessor());
+		ContextSnapshotFactory snapshotFactory = ContextSnapshotFactory.builder().contextRegistry(registry).build();
+
+		Runnable task = () -> actual.set(TestThreadLocalHolder.getValue());
+		TestThreadLocalHolder.setValue("expected");
+
+		Thread execution = new Thread(new ContextPropagatingTaskDecorator(snapshotFactory).decorate(task));
+		execution.start();
+		execution.join();
+		assertThat(actual.get()).isEqualTo("expected");
+		TestThreadLocalHolder.reset();
+	}
+
+	static class TestThreadLocalHolder {
+
+		private static final ThreadLocal<String> holder = new ThreadLocal<>();
+
+		public static void setValue(String value) {
+			holder.set(value);
+		}
+
+		public static String getValue() {
+			return holder.get();
+		}
+
+		public static void reset() {
+			holder.remove();
+		}
+
+	}
+
+	static class TestThreadLocalAccessor implements ThreadLocalAccessor<String> {
+
+		public static final String KEY = "test.threadlocal";
+
+		@Override
+		public Object key() {
+			return KEY;
+		}
+
+		@Override
+		public String getValue() {
+			return TestThreadLocalHolder.getValue();
+		}
+
+		@Override
+		public void setValue(String value) {
+			TestThreadLocalHolder.setValue(value);
+		}
+
+		@Override
+		public void setValue() {
+			TestThreadLocalHolder.reset();
+		}
+
+		@Override
+		public void restore(String previousValue) {
+			setValue(previousValue);
+		}
+
+	}
+
+}