I just read a description on Tor Metrics stating that: "These estimates are derived from the number of directory requests counted on directory authorities and mirrors". So my question is already stated above, but in more details: "does a directory-request get counted twice if the download from mirrors also counted?" Because as far as I am aware, all Tor clients only know the Directory Authorities, which are hard-coded into TBB, and have no idea about mirrors (directory-caches). In other words, I imply that all direct users first request the whole consensus document from Directory Authorities, but not the mirrors. Later, Client maintains the validity of the document by frequently fetching only descriptors (from the mirrors) which have different hash-value from the one that they are processing. By this way, the clients do not need to talk to Directory Authorities a second time. Please correct me if I imply something wrong. Best regards.
Every client that doesn't get its consensus from some other source (clients that use bridges get theirs from the bridge they connect to) will have to connect to the Directory Authorities once at first startup, and again if they're offline for a day or longer. Tor metrics should take this into account when calculating user numbers.