consider not counting hs directory bw as directory bytes written

bbb chat log:

arma: the dirbyte asymmetry can happen with many fresh tor clients bootstrapping over and over
trinity-1686a: fyi, i'm trying to add metrics to know if the asymmetry is many bootstrap, or fetching hs descriptor, or a bit of both
arma: trinity: i wonder if relays should include onion service descriptor writes in bwhist_note_dir_bytes_written(). that seems like maybe it should be private.
arma: (it looks from first glance like yes-they-do-include-it currently)
trinity-1686a: arma: it would be kinda annoying to ignore hs for the read count (from relay pov) because we don't know yet what it is when we do that part of the accounting
arma: but, for a relay that isn't a guard, it will serve very little *other* dir info besides hsdir info? maybe? that seems scary

cc @arma