add b3 header for trace propagation on initial webhook

lib/tasks/task.js

@@ -90,6 +90,13 @@ class Task extends Emitter {
     return {span, ctx};
   }
 
+  getTracingPropagation(encoding) {
+    // TODO: support encodings beyond b3 https://github.com/openzipkin/b3-propagation
+    if (this.span) {
+      return `${this.span.spanContext().traceId}-${this.span.spanContext().spanId}-1`;
+    }
+  }
+
   /**
    * when a subclass Task has completed its work, it should call this method
    */