figured it out.
/*
* Update last activity for a user
*/
add_action( 'init', 'update_session_activity' );
function update_session_activity() {
if ( is_user_logged_in() ) {
$manager = WP_Session_Tokens::get_instance( get_current_user_id() );
$token = wp_get_session_token();
$current_user_session = $manager->get( $token );
// update last activity to current server time
$current_user_session['last_activity'] = time();
// update current session with new activity time
$manager->update($token, $current_user_session);
}
}