Make the middleware create a new traceparent if no header was provided

lib/middleware.ts

@@ -1,5 +1,6 @@
 import type { RequestHandler } from "express";
 import { AsyncLocalStorage } from "node:async_hooks";
+import { createTraceparent } from "./traceparent";
 
 type Store = {
   traceparent?: string;
@@ -10,7 +11,7 @@ type Store = {
 const storage = new AsyncLocalStorage<Store>();
 
 export const middleware: RequestHandler = (req, _res, next) => {
-  const traceparent = req.header("traceparent");
+  const traceparent = req.header("traceparent") || createTraceparent();
 
   storage.run({ traceparent }, () => {
     next();

test/unit/middleware.test.ts

@@ -11,6 +11,17 @@ describe("Express middleware", () => {
     });
   });
 
+  it("should create a new traceparent and store it in the store if no traceparent header was provided", () => {
+    const req = { header: () => {} };
+
+    // @ts-expect-error - We don't need the full Express Request object
+    middleware(req, {}, () => {
+      const traceparent = getStore().traceparent; 
+      expect(traceparent).to.not.equal(undefined);
+      expect(traceparent.length).to.equal(55);
+    });
+  });
+
   it("should return an empty object if the middleware is not used", () => {
     expect(getStore()).to.deep.equal({});
   });