/**
 * Increment/decrement the value of a custom attribute. Only numeric custom attributes can be incremented. Attempts to
 * increment a custom attribute that is not numeric will be ignored. If you increment a custom attribute that has not
 * previously been set, a custom attribute will be created and assigned the value of incrementValue. To decrement the
 * value of a custom attribute, use a negative incrementValue.
 *
 * @param key - The identifier of the custom attribute. Limited to 255 characters in length, cannot begin with a $, and
 *   can only contain alphanumeric characters and punctuation.
 * @param incrementValue - Default 1. May be negative to decrement.
 *
 * @returns Resolves to a boolean that indicates whether the update was successfully enqueued. Resolves to undefined
 *   if the SDK is not initialized.
 */
export declare function incrementCustomUserAttribute(key: string, incrementValue?: number): Promise<boolean | undefined>;