Estimating fraction of reported directory-request statistics

@dcf has reported issues with how we are estimating reported directory-request statistics in https://lists.torproject.org/pipermail/tor-dev/2022-April/014724.html.

This estimation is currently happening in the sql scripts of the metrics website: https://gitlab.torproject.org/tpo/network-health/metrics/website/-/blob/master/src/main/sql/clients/init-userstats.sql#L695

We should document how we are estimating this and why.