chore(core): store cls contexts on a Map

Node.js v12 is affected by a V8 regression where many
insert/deletes on a POJO will trigger an undefined state on V8, leading
to a huge performance regression which can cause each insert/delete
operations on that object to take hundreds of times more. The Map
structure is not affected by this performance regression (and is
recommended by engine developers to deal with large key/value stores).

Contexts on cls-ah are stored on a POJO, and since inserts/deletes
happen often (they are triggered by async_hooks on init and destroy),
applications with high RPS or with high amount of async operations will
hit the threshold, turning the regression on and leading to a huge
performance overhead on each new async operation. Changing to a Map will
avoid that issue.

Ref: nodejs/node#31961

packages/opencensus-core/src/internal/cls-ah.ts

@@ -28,7 +28,7 @@ import * as shimmer from 'shimmer';
 
 const WRAPPED = Symbol('context_wrapped');
 /** A map of AsyncResource IDs to Context objects. */
-let contexts: { [asyncId: number]: Context } = {};
+let contexts: Map<number, Context> = new Map();
 let current: Context = {};
 
 // Create the hook.
@@ -137,13 +137,14 @@ function init(
   parentUid: number,
   parentHandle: {}
 ) {
-  contexts[uid] = current;
+  contexts.set(uid, current);
 }
 
 /** before is called just before the resource's callback is called. */
 function before(uid: number) {
-  if (contexts[uid]) {
-    current = contexts[uid];
+  const maybeCurrent = contexts.get(uid);
+  if (maybeCurrent !== undefined) {
+    current = maybeCurrent;
   }
 }
 
@@ -152,7 +153,7 @@ function before(uid: number) {
  * its entry in the map.
  */
 function destroy(uid: number) {
-  delete contexts[uid];
+  contexts.delete(uid);
 }
 
 export function createNamespace(): CLSNamespace {
@@ -161,7 +162,7 @@ export function createNamespace(): CLSNamespace {
 
 export function destroyNamespace(): void {
   current = {};
-  contexts = {};
+  contexts = new Map();
 }
 
 export function getNamespace(): CLSNamespace {