Document how Grafana time series powered by VictoriaMetrics work by giving an example

I've been looking pretty closely at different steps for panels when doing a dashboard for https://gitlab.torproject.org/tpo/network-health/analysis/-/issues/81 as the steps results in different numbers/results and it was not obvious to me how this works at first glance.

Gonna add some sentences to METRICS.md to help others avoiding the "reverse-engineering" effort.