avoid using ExecutionContextImpl on executor advice

instrumentation/kamon-executors/src/main/scala-2.12/kamon/instrumentation/executor/ScalaGlobalExecutionContextAdvice.java

@@ -20,15 +20,20 @@
 import scala.concurrent.ExecutionContext;
 import scala.concurrent.impl.ExecutionContextImpl;
 
+import java.lang.reflect.Method;
 import java.util.concurrent.ExecutorService;
 
 import static kanela.agent.libs.net.bytebuddy.implementation.bytecode.assign.Assigner.Typing.DYNAMIC;
 
 final class ScalaGlobalExecutionContextAdvice {
 
     @Advice.OnMethodExit
-    public static void onExit(@Advice.Return(readOnly = false, typing = DYNAMIC) ExecutionContextImpl returnValue) {
-        ExecutorService instrumented = ExecutorInstrumentation.instrument((ExecutorService) returnValue.executor(), "scala-global-execution-context");
+    public static void onExit(@Advice.Return(readOnly = false, typing = DYNAMIC) Object returnValue) throws Exception {
+        // Not ideal to go through reflection but this code will only be executed once in the lifetime of the JVM
+        Method executorMethod = returnValue.getClass().getDeclaredMethod("executor");
+        ExecutorService executor = (ExecutorService) executorMethod.invoke(returnValue);
+
+        ExecutorService instrumented = ExecutorInstrumentation.instrument(executor, "scala-global-execution-context");
         returnValue = new ExecutionContextImpl(instrumented, ExecutionContext.defaultReporter());
     }
 }