Do not inspect reactor context when not needed

dd-java-agent/instrumentation/reactor-core-3.1/src/main/java/datadog/trace/instrumentation/reactor/core/ContextSpanHelper.java

@@ -4,16 +4,37 @@
 import datadog.trace.bootstrap.instrumentation.api.WithAgentSpan;
 import javax.annotation.Nullable;
 import reactor.core.CoreSubscriber;
+import reactor.core.publisher.Mono;
 import reactor.util.context.Context;
 
 public class ContextSpanHelper {
+
+  private static final Class<?> MONO_WITH_CONTEXT_CLASS = findMonoWithContextClass();
+
   private static final String DD_SPAN_KEY = "dd.span";
 
+  private static Class<?> findMonoWithContextClass() {
+    final ClassLoader classLoader = Mono.class.getClassLoader();
+    // 3.4+
+    try {
+      return Class.forName(
+          "reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber", false, classLoader);
+    } catch (Throwable ignored) {
+    }
+    // < 3.4
+    try {
+      return Class.forName(
+          "reactor.core.publisher.FluxContextStart$ContextStartSubscriber", false, classLoader);
+    } catch (Throwable ignored) {
+    }
+    return null;
+  }
+
   private ContextSpanHelper() {}
 
   @Nullable
   public static AgentSpan extractSpanFromSubscriberContext(final CoreSubscriber<?> subscriber) {
-    if (subscriber == null) {
+    if (MONO_WITH_CONTEXT_CLASS == null || !MONO_WITH_CONTEXT_CLASS.isInstance(subscriber)) {
       return null;
     }
     Context context = null;