use AsyncLocalStorage instead of our home-grown solutions (#4201)

* use AsyncLocalStorage instead of our home-grown solutions

The comment in the file that selected a storage implementation suggested
just using AsyncLocalStorage once it supports triggerAsyncResource().
That said, literally zero of our code uses triggerAsyncResource(), so
this is assumed to be historical and no longer relevant.

Switching to stock AsyncLocalStorage will enable the usage of
TracingChannel in the future.

* self-contain profiling's AsyncLocalStorage channel usage

* remove flag detection

packages/datadog-core/index.js

@@ -1,7 +1,7 @@
 'use strict'
 
-const LocalStorage = require('./src/storage')
+const { AsyncLocalStorage } = require('async_hooks')
 
-const storage = new LocalStorage()
+const storage = new AsyncLocalStorage()
 
 module.exports = { storage }

packages/dd-trace/src/profiling/profilers/wall.js

@@ -76,6 +76,44 @@ function getWebTags (startedSpans, i, span) {
   return memoize(null)
 }
 
+let channelsActivated = false
+function ensureChannelsActivated () {
+  if (channelsActivated) return
+
+  const { AsyncLocalStorage, createHook } = require('async_hooks')
+  const shimmer = require('../../../../datadog-shimmer')
+
+  createHook({ before: () => beforeCh.publish() }).enable()
+
+  let inRun = false
+  shimmer.wrap(AsyncLocalStorage.prototype, 'enterWith', function (original) {
+    return function (...args) {
+      const retVal = original.apply(this, args)
+      if (!inRun) enterCh.publish()
+      return retVal
+    }
+  })
+
+  shimmer.wrap(AsyncLocalStorage.prototype, 'run', function (original) {
+    return function (store, callback, ...args) {
+      const wrappedCb = shimmer.wrapFunction(callback, cb => function (...args) {
+        inRun = false
+        enterCh.publish()
+        const retVal = cb.apply(this, args)
+        inRun = true
+        return retVal
+      })
+      inRun = true
+      const retVal = original.call(this, store, wrappedCb, ...args)
+      enterCh.publish()
+      inRun = false
+      return retVal
+    }
+  })
+
+  channelsActivated = true
+}
+
 class NativeWallProfiler {
   constructor (options = {}) {
     this.type = 'wall'
@@ -121,6 +159,8 @@ class NativeWallProfiler {
   start ({ mapper } = {}) {
     if (this._started) return
 
+    ensureChannelsActivated()
+
     this._mapper = mapper
     this._pprof = require('@datadog/pprof')
     kSampleCount = this._pprof.time.constants.kSampleCount