bug#32548: Cuirass: Performance monitoring

[Mathieu Othacehe]

Hello,

I just pushed support for computing and displaying metrics in Cuirass. I
started with two metrics:

* Builds per day
* Average evaluation speed per specification.

Those metrics can now be seen at:

https://ci.guix.gnu.org/metrics

and are updated every hour.

I plan to add more metrics such as:

- Evaluation completion percentage.
- Evaluation completion speed.
- Failed evaluations percentage.
- Pending builds per day.

Don't hesitate to comment or propose other metrics.

Thanks,

Mathieu