process: add 'worker' event

Provides a new `process.on('worker', (worker) => {})` event that
is triggered by the creation of a new `worker_thread.Worker`.
The use case is to allow hooks to be installed for monitoring
workers without having to modify the call sites around those.

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #38659
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>

Author: jasnell

doc/api/process.md

@@ -455,6 +455,15 @@ of the custom deprecation.
 The `*-deprecation` command-line flags only affect warnings that use the name
 `'DeprecationWarning'`.
 
+### Event: `'worker'`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `worker` {Worker} The {Worker} that was created.
+
+The `'worker'` event is emitted after a new {Worker} thread has been created.
+
 #### Emitting custom warnings
 
 See the [`process.emitWarning()`][process_emit_warning] method for issuing

lib/internal/worker.js

@@ -261,6 +261,8 @@ class Worker extends EventEmitter {
     };
     // Actually start the new thread now that everything is in place.
     this[kHandle].startThread();
+
+    process.nextTick(() => process.emit('worker', this));
   }
 
   [kOnExit](code, customErr, customErrReason) {

test/parallel/test-worker-event.js

@@ -0,0 +1,13 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const {
+  Worker,
+} = require('worker_threads');
+
+process.on('worker', common.mustCall(({ threadId }) => {
+  assert.strictEqual(threadId, 1);
+}));
+
+new Worker('', { eval: true });