Add page speed/loading/web-vitals metrics

**🐣 Is your feature request related to a problem? Please describe.**

It would be nice to track the load times of different pages. This would allow you to see what pages take longer and might need to be improved or how the load times vary by load/usage or after a new version is deployed.

**💡 Describe the solution you'd like**

Using the [ackee-tracker](https://github.com/electerious/ackee-tracker) you could get the time it took to load the page like this:

```js
const loadTime = window.performance.timing.domContentLoadedEventEnd- window.performance.timing.navigationStart;
```

And then just send it along with all the other data to the server.

Ackee could display it similarly to other metrics like pages. Here's a quick mockup:


Or even better show you the loading times of different pages over time, similarly to how durations are displayed.

**📋 Additional context**

[Here's](https://app.kindmetrics.io/share/851Jv1PW/page_speeds) how [Kindmetrics](kindmetrics.io) displays this. They also show you page speeds grouped by country which I think is very useful too, but as you mentioned in #103 this can't be easily integrated with Ackee.