src/vs/platform/telemetry/node/appInsightsAppender.ts/AppInsightsAppender/constructor
class AppInsightsAppender:	constructor(
		private _eventPrefix: string,
		private _defaultData: { [key: string]: any } | null,
		aiKeyOrClientFactory: string | (() => appInsights.TelemetryClient), // allow factory function for testing
	) {
		if (!this._defaultData) {
			this._defaultData = Object.create(null);
		}

		if (typeof aiKeyOrClientFactory === 'function') {
			this._aiClient = aiKeyOrClientFactory();
		} else {
			this._aiClient = aiKeyOrClientFactory;
		}
		this._asyncAIClient = null;
	}