hey when i follow the official example to add Google statistics in my nuxt.js pro, console prompt me 'ga is not defined',What did I miss？
<!--cmty--><!--cmty_prevent_hook--><div align="right"><sub><em>This bug report is available on <a href="https://nuxtjs.cmty.io">Nuxt.js</a> community (<a href="https://nuxtjs.cmty.io/nuxt/nuxt.js/issues/c1631">#c1631</a>)</em></sub></div>