include trace ID in each downstream request

middleware/trace.js

@@ -3,5 +3,9 @@ const uuid = require('uuid/v1');
 // Trace middleware attaches a request id to each incoming request.
 module.exports = (req, res, next) => {
   req.id = uuid();
+
+  // Add the context ID to the request as an HTTP header.
+  res.set('X-Talk-Trace-ID', req.id);
+
   next();
 };