Refine PropagationContextElement

This commit apply several refinements to PropagationContextElement:
 - Capture the ThreadLocal when instantiating the
   PropagationContextElement in order to support dispatchers switching
   threads
 - Remove the constructor parameter which is not idiomatic and breaks
   the support when switching threads, and use instead the
   updateThreadContext(context: CoroutineContext) parameter
 - Make the kotlinx-coroutines-reactor dependency optional
 - Make the properties private

The Javadoc and tests are also updated to use the
`Dispatchers.IO + PropagationContextElement()` pattern performed
outside of the suspending lambda, which is the typical use case.

Closes gh-35469

framework-docs/modules/ROOT/pages/languages/kotlin/coroutines.adoc

@@ -257,29 +257,20 @@ For Kotlin `Flow`, a  `Flow<T>.transactional` extension is provided.
 Spring applications are xref:integration/observability.adoc[instrumented with Micrometer for Observability support].
 For tracing support, the current observation is propagated through a `ThreadLocal` for blocking code,
 or the Reactor `Context` for reactive pipelines. But the current observation also needs to be made available
-in the execution context of a suspended function. Without that, the current "traceId" will not be automatically prepended
-to logged statements from coroutines.
+in the execution context of a suspended function. Without that, the current "traceId" will not be automatically
+prepended to logged statements from coroutines.
 
-The `org.springframework.core.PropagationContextElement` operator generally ensures that the
+The {spring-framework-api-kdoc}/spring-core/org.springframework.core/-propagation-context-element/index.html[`PropagationContextElement`] operator generally ensures that the
 {micrometer-context-propagation-docs}/[Micrometer Context Propagation library] works with Kotlin Coroutines.
 
-The `PropagationContextElement` requires the following dependencies:
+It requires the `io.micrometer:context-propagation` dependency and optionally the
+`org.jetbrains.kotlinx:kotlinx-coroutines-reactor` one.
 
-`build.gradle.kts`
-[source,kotlin,indent=0]
-----
-dependencies {
-    implementation("io.micrometer:context-propagation:${contextPropagationVersion}")
-	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:${coroutinesVersion}")
-	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor:${coroutinesVersion}")
-}
-----
-
-Applications can then use the `PropagationContextElement` operator to connect the `currentCoroutineContext()`
+Applications can then use the `PropagationContextElement` to augment the `CoroutineContext`
 with the context propagation mechanism:
 
 include-code::./ContextPropagationSample[tag=context,indent=0]
 
-Here, assuming that Micrometer Tracing is configured, the resulting logging statement
-will show the current "traceId" and unlock better observability for your application.
+Here, assuming that Micrometer Tracing is configured, the resulting logging statement will show the current "traceId"
+and unlock better observability for your application.
 

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

@@ -16,8 +16,9 @@
 
 package org.springframework.docs.languages.kotlin.coroutines.propagation
 
-import kotlinx.coroutines.currentCoroutineContext
-import kotlinx.coroutines.withContext
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.runBlocking
 import org.apache.commons.logging.Log
 import org.apache.commons.logging.LogFactory
 import org.springframework.core.PropagationContextElement
@@ -31,10 +32,15 @@ class ContextPropagationSample {
 	}
 
 	// tag::context[]
+	fun main() {
+		runBlocking(Dispatchers.IO + PropagationContextElement()) {
+			suspendingFunction()
+		}
+	}
+
 	suspend fun suspendingFunction() {
-		return withContext(PropagationContextElement(currentCoroutineContext())) {
-            logger.info("Suspending function with traceId")
-        }
+		delay(1)
+		logger.info("Suspending function with traceId")
 	}
 	// end::context[]
 }