message: Conditionally initialize ScopeManager
  To avoid async_hooks being used by default even when performanceStats is disabled, because it comes with performance overhead.
diff: @@ -5,8 +5,8 @@ import { Scope, ScopeManager } from './scope';
  export class Notifier extends BaseNotifier {
  _inFlight: number;
  - _scopeManager: ScopeManager;
  - _mainScope: Scope;
  + _scopeManager?: ScopeManager;
  + _mainScope?: Scope;
  constructor(opt: IOptions) {
  if (!opt.environment && process.env.NODE_ENV) {
  @@ -18,8 +18,6 @@ export class Notifier extends BaseNotifier {
  this.addFilter(nodeFilter);
  this._inFlight = 0;
  - this._scopeManager = new ScopeManager();
  - this._mainScope = new Scope();
  process.on('beforeExit', async () => {
  await this.flush();
  @@ -54,14 +52,19 @@ export class Notifier extends BaseNotifier {
  if (opt.performanceStats) {
  this._instrument();
  + this._scopeManager = new ScopeManager();
  }
  +
  + this._mainScope = new Scope();
  }
  scope(): Scope {
  - let scope = this._scopeManager.active();
  + if (this._scopeManager) {
  + const scope = this._scopeManager.active();
  if (scope) {
  return scope;
  }
  + }
  return this._mainScope;
  }
  