Allow reactor instrumentation to pick up spans in reactor context in …

…addition to current tracing context

instrumentation/reactor-3.1/library/src/main/java/io/opentelemetry/instrumentation/reactor/TracingSubscriber.java

@@ -31,6 +31,7 @@
  * https://github.com/opentracing-contrib/java-reactor/blob/master/src/main/java/io/opentracing/contrib/reactor/TracedSubscriber.java
  */
 public class TracingSubscriber<T> implements CoreSubscriber<T> {
+  public final static String EXPLICIT_SPAN_KEY = "otel-span-key";
   private final io.opentelemetry.context.Context traceContext;
   private final Subscriber<? super T> subscriber;
   private final Context context;
@@ -45,7 +46,13 @@ public TracingSubscriber(
       io.opentelemetry.context.Context contextToPropagate) {
     this.subscriber = subscriber;
     this.context = ctx;
-    this.traceContext = contextToPropagate;
+
+    var span = (Span)this.context.getOrDefault(EXPLICIT_SPAN_KEY, null);
+    if (span != null) {
+      this.traceContext = contextToPropagate.with(span);
+    } else {
+      this.traceContext = contextToPropagate;
+    }
   }
 
   @Override