src/vs/platform/telemetry/common/1dsAppender.ts/AbstractOneDataSystemAppender/constructor
class AbstractOneDataSystemAppender:	constructor(
		private readonly _isInternalTelemetry: boolean,
		private _eventPrefix: string,
		private _defaultData: { [key: string]: any } | null,
		iKeyOrClientFactory: string | (() => IAppInsightsCore), // allow factory function for testing
		private _xhrOverride?: IXHROverride
	) {
		if (!this._defaultData) {
			this._defaultData = {};
		}

		if (typeof iKeyOrClientFactory === 'function') {
			this._aiCoreOrKey = iKeyOrClientFactory();
		} else {
			this._aiCoreOrKey = iKeyOrClientFactory;
		}
		this._asyncAiCore = null;
	}