docs: add reactor context propagation example

src/main/docs/guide/contextPropagation/reactorContextPropagation.adoc

@@ -18,6 +18,13 @@ After that, all the thread-local propagated elements can restore their thread-lo
 
 NOTE: The thread-local values are read-only. To modify them, the `PropagatedContext` instance needs to be changed and put into the Reactor's context.
 
+To add the context in the middle of the reactive chain, do something like the following:
+
+snippet::io.micronaut.docs.propagation.reactor.HelloController[tags="imports,example", indent=0]
+
+<1> Obtain the current context that has been modified, e.g. with `PropagatedContext.get().plus(...)`, etc.
+<2> Add the context into the reactive chain.
+
 If you have Micrometer Context Propagation on the classpath but don't want to use it, apply the following configuration:
 
 .Disable Micrometer Context Propagation in Reactor

test-suite-groovy/src/test/groovy/io/micronaut/docs/propagation/reactor/HelloController.groovy

@@ -0,0 +1,28 @@
+package io.micronaut.docs.propagation.reactor
+
+// tag::imports[]
+import io.micronaut.core.async.propagation.ReactorPropagation
+import io.micronaut.core.propagation.PropagatedContext
+import io.micronaut.core.propagation.PropagatedContextElement
+import io.micronaut.http.annotation.Controller
+import io.micronaut.http.annotation.Get
+import io.micronaut.http.annotation.QueryValue
+import reactor.core.publisher.Mono
+// end::imports[]
+import io.micronaut.context.annotation.Requires
+
+@Requires(property = 'spec.name', value = 'PropagatedContextSpec')
+// tag::example[]
+@Controller
+class HelloController {
+
+    @Get('/hello')
+    Mono<String> hello(@QueryValue('name') String name) {
+        PropagatedContext propagatedContext = PropagatedContext.get() + new MyContextElement(name) // <1>
+        return Mono.just("Hello, $name")
+                .contextWrite(ctx -> ReactorPropagation.addPropagatedContext(ctx, propagatedContext)) // <2>
+    }
+}
+
+record MyContextElement(String value) implements PropagatedContextElement { }
+// end::example[]

test-suite-groovy/src/test/groovy/io/micronaut/docs/propagation/reactor/PropagatedContextSpec.groovy

@@ -0,0 +1,29 @@
+package io.micronaut.docs.propagation.reactor
+
+import io.micronaut.context.annotation.Property
+import io.micronaut.core.type.Argument
+import io.micronaut.http.HttpRequest
+import io.micronaut.http.client.HttpClient
+import io.micronaut.http.client.annotation.Client
+import io.micronaut.http.uri.UriBuilder
+import io.micronaut.test.extensions.spock.annotation.MicronautTest
+import jakarta.inject.Inject
+import spock.lang.Specification
+
+@Property(name = 'spec.name', value = 'PropagatedContextSpec')
+@MicronautTest
+class PropagatedContextSpec extends Specification {
+
+    @Inject
+    @Client("/")
+    HttpClient client
+
+    void "test Mono Request"() {
+        when:
+        URI uri = UriBuilder.of('/hello').queryParam('name', 'Dean').build()
+        String hello = client.toBlocking().retrieve(HttpRequest.GET(uri), Argument.of(String))
+
+        then:
+        'Hello, Dean' == hello
+    }
+}

test-suite/src/test/java/io/micronaut/docs/propagation/reactor/HelloController.java

@@ -0,0 +1,28 @@
+package io.micronaut.docs.propagation.reactor;
+
+// tag::imports[]
+import io.micronaut.core.async.propagation.ReactorPropagation;
+import io.micronaut.core.propagation.PropagatedContext;
+import io.micronaut.core.propagation.PropagatedContextElement;
+import io.micronaut.http.annotation.Controller;
+import io.micronaut.http.annotation.Get;
+import io.micronaut.http.annotation.QueryValue;
+import reactor.core.publisher.Mono;
+// end::imports[]
+import io.micronaut.context.annotation.Requires;
+
+@Requires(property = "spec.name", value = "PropagatedContextSpec")
+// tag::example[]
+@Controller
+class HelloController {
+
+    @Get("/hello")
+    Mono<String> hello(@QueryValue("name") String name) {
+        PropagatedContext propagatedContext = PropagatedContext.get().plus(new MyContextElement(name)); // <1>
+        return Mono.just("Hello, " + name)
+            .contextWrite(ctx -> ReactorPropagation.addPropagatedContext(ctx, propagatedContext)); // <2>
+    }
+}
+
+record MyContextElement(String value) implements PropagatedContextElement { }
+// end::example[]

test-suite/src/test/java/io/micronaut/docs/propagation/reactor/PropagatedContextTest.java

@@ -0,0 +1,31 @@
+package io.micronaut.docs.propagation.reactor;
+
+import io.micronaut.context.annotation.Property;
+import io.micronaut.core.type.Argument;
+import io.micronaut.http.HttpRequest;
+import io.micronaut.http.client.HttpClient;
+import io.micronaut.http.client.annotation.Client;
+import io.micronaut.http.uri.UriBuilder;
+import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
+import jakarta.inject.Inject;
+import org.junit.jupiter.api.Test;
+
+import java.net.URI;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@Property(name = "spec.name", value = "PropagatedContextSpec")
+@MicronautTest
+class PropagatedContextTest {
+
+    @Inject
+    @Client("/")
+    HttpClient client;
+
+    @Test
+    void testMonoRequest() {
+        URI uri = UriBuilder.of("/hello").queryParam("name", "Dean").build();
+        String hello = client.toBlocking().retrieve(HttpRequest.GET(uri), Argument.of(String.class));
+        assertEquals("Hello, Dean", hello);
+    }
+}