Fix demo dashboards

[olegbespalov]

What?

These project demo dashboards initially were copied from the xk6-output-promete project. The Prometheus output extension treats some metrics in a certain Prometheus way see mapping, for instance.

So, currently, the dashboards don't work correctly.

We have to adjust them accordingly and make them fully work.

Why?

Having working examples (dashboards) is essential for our users. Making the dashboards work is also an excellent exercise for proving that the output works correctly.