Make a md file about reproducible metrics

On metrics.tpo we have a page about reproducible metrics, where it is explained how it is possible to reproduce the graphs that are published on the website (https://metrics.torproject.org/reproducible-metrics.html). The same graphs can be obtained in grafana or via querying victoriametrics directly, but people need to experiment a bit and understand how to extract data from timeseries.

We should have a file to explain how to recreate that aggregated data from timeseries on VM.