send telemetry when an update is available

src/vs/platform/update/electron-main/updateService.ts

@@ -177,6 +177,7 @@ export class UpdateService implements IUpdateService {
 				this._availableUpdate = data;
 				this._onUpdateAvailable.fire({ url: update.url, version: update.version });
 				this.state = State.UpdateAvailable;
+				this.telemetryService.publicLog('update:available', { explicit, version: update.version, currentVersion: product.commit });
 
 			} else {
 				const data: IUpdate = {