<div>
    <input type="checkbox" name="toggleGoogleAnalytics" id="toggleGoogleAnalytics">
    <label for="toggleGoogleAnalytics">Allow Google Analytics to track my visit.</label>
</div>

<script>
    var gaProperty = 'YOUR-ANALYTICS-PROPERTY';
    var gaDisableCookieName = 'ga-disable-' + gaProperty;
    var gaIsAllowed = document.cookie.indexOf(gaDisableCookieName + '=true') === -1
    var gaCheckbox = document.getElementById('toggleGoogleAnalytics');
    gaCheckbox.checked = gaIsAllowed;

    var gaOptout = function () {
        document.cookie = gaDisableCookieName + '=true; expires=Thu, 31 Dec 2099 23:59:59 UTC; path=/';
    }

    var gaOptin = function () {
        document.cookie = gaDisableCookieName + '=; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/';
    }

    gaCheckbox.addEventListener('change', function () {
        if (this.checked === true) {
            gaOptin();
        } else {
            gaOptout();
        }
    });
</script>