使用 gtag.js 增加按鈕事件

常常被客戶要求 Google Analytics 要設定按鈕事件，所以先筆記起來一下

<a class="doc-dl xls" title="下載 Excel" href="/TC/OPENDATA/Comm1_MEMBER.xls">Excel</a>
<a class="doc-dl pdf" title="下載 PDF" href="/TC/OPENDATA/Comm1_MEMBER.pdf">pdf</a>
<a class="doc-dl csv" title="下載 CSV" href="TC/OPENDATA/Comm1_MEMBER.csv">csv</a>
<a class="doc-dl xml" title="下載 XML" href="/TC/OPENDATA/Comm1_MEMBER.xml">xml</a>
<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=追蹤碼"></script>
<script>
    window.dataLayer = window.dataLayer || [];
    function gtag() {
        dataLayer.push(arguments);
    }
    gtag('js', new Date());
    gtag('config', '追蹤碼');

    function download_tracking(e) {
        var addressValue = e.toString();
        addressValue = addressValue.replace('網站domain', '');
        gtag('event', '下載', {
           'event_category': 'OpenData',
           'event_label': addressValue
    });
}
</script>