Add PropagationContextElement Kotlin operator

Prior to this commit, the Micrometer context-propagation project would
help propagating information from `ThreadLocal`, Reactor `Context` and
other context objects. This is already well supported for Micrometer
Observations.

In the case of Kotlin suspending functions, the processing of tasks
would not necessarily update the `ThreadLocal` when the function is
scheduled on a different thread.

This commit introduces the `PropagationContextElement` operator that
connects the `ThreadLocal`, Reactor `Context` and Coroutine `Context`
for all libraries using the "context-propagation" project.

Applications must manually use this operator in suspending functions
like so:

```
suspend fun suspendingFunction() {
  return withContext(PropagationContextElement(currentCoroutineContext())) {
    logger.info("Suspending function with traceId")
  }
}
```

Closes gh-35185

framework-docs/src/main/kotlin/org/springframework/docs/languages/kotlin/coroutines/propagation/ContextPropagationSample.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2002-present 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.languages.kotlin.coroutines.propagation
+
+import kotlinx.coroutines.currentCoroutineContext
+import kotlinx.coroutines.withContext
+import org.apache.commons.logging.Log
+import org.apache.commons.logging.LogFactory
+import org.springframework.core.PropagationContextElement
+
+class ContextPropagationSample {
+
+	companion object {
+		private val logger: Log = LogFactory.getLog(
+			ContextPropagationSample::class.java
+		)
+	}
+
+	// tag::context[]
+	suspend fun suspendingFunction() {
+		return withContext(PropagationContextElement(currentCoroutineContext())) {
+            logger.info("Suspending function with traceId")
+        }
+	}
+	// end::context[]
+}