/**
 * Filters request headers to extract only custom headers that are safe to forward.
 * Blocked headers and non-string values are excluded.
 *
 * @param headers - The raw incoming request headers.
 * @returns The subset of headers that are safe to forward to the Admin API.
 */
export declare function filterCustomHeaders(headers: {
    [key: string]: string | string[] | undefined;
}): {
    [key: string]: string;
};