fix: add required span.parent_id

Author: watson

lib/instrumentation/span.js

@@ -149,6 +149,7 @@ Span.prototype._encode = function (cb) {
     var payload = {
       id: self.id,
       transaction_id: self.transaction.id,
+      parent_id: self.transaction.id,
       trace_id: self.transaction.traceId,
       timestamp: self.transaction.timestamp,
       name: self.name,