System metrics should be plotted with `timestamp` rather than `step`

[AlexandreKempf]

The system metrics should be plotted with timestamp rather than step.

TODO list:

• [ ] Live should save in the dvcyaml:

metrics:
- dvclive/metrics.json
plots:
- dvclive/plots/metrics:
    x: step
- dvclive/plots/metrics/system:
    x: timestamp

• [ ] Add a new template to display relative timestamp (timestamp axis that start at 0).

Blocker: https://github.com/iterative/dvc/pull/10320

[observingClouds]

Hi! 👋 Is there any update on this? I am trying to get the system parameters visualized in VSCode, but every time I set x: timestamp it gets deleted when I run another experiment (dvc run exp).