feat: add diagnostics_channel support for app initialization

lib/diagnostics.js

@@ -0,0 +1,7 @@
+'use strict';
+
+const dc = require('node:diagnostics_channel');
+
+const initialization = dc.channel('express.initialization');
+
+module.exports = { initialization };

lib/express.js

@@ -19,6 +19,7 @@ var proto = require('./application');
 var Router = require('router');
 var req = require('./request');
 var res = require('./response');
+const channels = require('./diagnostics');
 
 /**
  * Expose `createApplication()`.
@@ -52,6 +53,11 @@ function createApplication() {
   })
 
   app.init();
+
+  if (channels.initialization.hasSubscribers) {
+    channels.initialization.publish({ app });
+  }
+
   return app;
 }
 

test/diagnostics-channel.js

@@ -0,0 +1,26 @@
+'use strict'
+
+const dc = require('node:diagnostics_channel')
+const express = require('../')
+const assert = require('node:assert')
+
+describe('diagnostics channels', function () {
+  describe('express.initialization', function () {
+    it('should publish when app is created', function () {
+      let msg
+
+      dc.subscribe('express.initialization', function handler (data) {
+        msg = data
+        dc.unsubscribe('express.initialization', handler)
+      })
+
+      const app = express()
+      assert.ok(msg, 'express.initialization was not published')
+      assert.strictEqual(msg.app, app)
+    })
+
+    it('should not throw when there are no subscribers', function () {
+      assert.ok(express())
+    })
+  })
+})