Propagate client span context in doOnRequest (open-telemetry#6621)

Copying the changes from Mike W. This fixes the problem where the jaxrs
and http client request spans are siblings instead of parent/child.

@mateuszrzeszutek please review

Co-authored-by: Trask Stalnaker <trask.stalnaker@gmail.com>

instrumentation/reactor/reactor-netty/reactor-netty-1.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/reactornetty/v1_0/HttpClientInstrumentation.java

@@ -85,7 +85,9 @@ public static void onEnter(
             BiConsumer<? super HttpClientRequest, ? super Connection> callback) {
 
       if (DecoratorFunctions.shouldDecorate(callback.getClass())) {
-        callback = new DecoratorFunctions.OnMessageDecorator<>(callback, PropagatedContext.PARENT);
+        // perform the callback with the client span active (instead of the parent) since this
+        // callback occurs after the connection is made
+        callback = new DecoratorFunctions.OnMessageDecorator<>(callback, PropagatedContext.CLIENT);
       }
     }
   }

instrumentation/reactor/reactor-netty/reactor-netty-1.0/javaagent/src/test/java/io/opentelemetry/javaagent/instrumentation/reactornetty/v1_0/AbstractReactorNettyHttpClientTest.java

@@ -180,7 +180,7 @@ void shouldExposeContextToHttpClientCallbacks() throws InterruptedException {
               span -> span.hasName("HTTP GET").hasKind(CLIENT).hasParent(parentSpan),
               span -> span.hasName("test-http-server").hasKind(SERVER).hasParent(nettyClientSpan));
 
-          assertSameSpan(parentSpan, onRequestSpan);
+          assertSameSpan(nettyClientSpan, onRequestSpan);
           assertSameSpan(nettyClientSpan, afterRequestSpan);
           assertSameSpan(nettyClientSpan, onResponseSpan);
           assertSameSpan(parentSpan, afterResponseSpan);