Added addition metrics for dynamic dispatches

Via RequestObserver, workers can report that they are actively dispatching a subpipeline (`setHasDispatchBinding()`), and target workers can report what worker dispatched them (`setDispatcherId()`)

src/workerd/io/observer.h

@@ -62,6 +62,12 @@ class RequestObserver: public kj::Refcounted {
     return kj::mv(client);
   }
 
+  // Used to record dynamic dispatch workers.
+  virtual void setHasDispatchBinding() {};
+
+  // Used to record what worker dispatched the current worker.
+  virtual void setDispatcherId(kj::Maybe<kj::String> id) {};
+
   virtual SpanParent getSpan() { return nullptr; }
 
   virtual void addedContextTask() {}