Fix graph titles in `Torrust Live Demo Tracker (metrics)` Grafana dashboard

[josecelano]

In the new Grafana dashboard, the graphs include both UDP4 and UDP6 requests.

We have two options:

1. Rename graph titles.
2. Create a new dashboard for UDP6 and filter by protocol label. Like this:

NOTE: IPV6 is not enabled yet in the demo. So the UDP 4 titles are correct for now.

I suggest creating a new one for UDP6.

[josecelano]

I'm adding a new label to the graphs to make sure data are only for UDP4:

When we support inet6 I will create a new dashboard.

[josecelano]

Graph's titles changed:

[josecelano]

Fixed via #81