add diagnostics_channel publish at load time

fastify.js

@@ -707,3 +707,11 @@ function loadVersion () {
 module.exports = fastify
 module.exports.fastify = fastify
 module.exports.default = fastify
+
+try {
+  const dc = require('diagnostics_channel')
+  dc.channel('fastify:loaded').publish(fastify)
+} catch (e) {
+  // This only happens if `diagnostics_channel` isn't available, i.e. earlier versions of Node.js.
+  // In that event, we don't care, so ignore the error.
+}

test/diagnostics-channel.test.js

@@ -0,0 +1,19 @@
+'use strict'
+
+const t = require('tap')
+const test = t.test
+const dc = require('diagnostics_channel')
+
+test('diagnostics_channel', t => {
+  t.plan(2)
+
+  let fastifyInHook
+
+  const channel = dc.channel('fastify:loaded')
+  channel.subscribe(function (fastify) {
+    t.pass('message passed to channel')
+    fastifyInHook = fastify
+  })
+  const fastify = require('../fastify')
+  t.equal(fastifyInHook, fastify)
+})