add diagnostics_channel publish at initializaiton time

fastify.js

@@ -399,6 +399,14 @@ function fastify (options) {
   // Delay configuring clientError handler so that it can access fastify state.
   server.on('clientError', options.clientErrorHandler.bind(fastify))
 
+  try {
+    const dc = require('diagnostics_channel')
+    dc.channel('fastify:initialized').publish(fastify)
+  } catch {
+    // 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.
+  }
+
   return fastify
 
   function throwIfAlreadyStarted (msg) {

test/diagnostics-channel.test.js

@@ -0,0 +1,29 @@
+'use strict'
+
+const t = require('tap')
+const test = t.test
+let dc
+try {
+  dc = require('diagnostics_channel')
+} catch {
+  // Not available in this version of Node.js
+}
+
+test('diagnostics_channel', t => {
+  if (!dc) {
+    t.plan(1)
+    t.skip('diagnostics_channel is not available')
+    return
+  }
+  t.plan(2)
+
+  let fastifyInHook
+
+  const channel = dc.channel('fastify:initialized')
+  channel.subscribe(function (fastify) {
+    t.pass('message passed to channel')
+    fastifyInHook = fastify
+  })
+  const fastify = require('../fastify')()
+  t.equal(fastifyInHook, fastify)
+})