Move metric definitions to YAML - System Metrics

[joaopgrassi]

Move the metric definitions text from system-metrics.md to a respective system-metrics.yaml and re-generate the markdown tables.

[frzifus]

I can work on this

[joaopgrassi]

Hi @frzifus! I sent a PR for it some time ago #89. I think what is missing is define/agree on the attribute namespaces and how we will communicate the probably breaking change in them. If you have time feel free to take over!