HSDirs should publish some count about new-style onion addresses (v3 metrics)

Right now we have an ongoing estimate of the total number of onion addresses published to the HSDirs: https://metrics.torproject.org/hidserv-dir-onions-seen.html

How many of those are 224-style onion addresses, and how many of them are legacy-style onion addresses?

I see a rep_hist_stored_maybe_new_hs() for the v2-style descriptors, and I think I see a

  /* XXX: Update HS statistics. We should have specific stats for v3. */

for the v3-style descriptors.

So I think that means that the graph is only showing v2-style onions, and we have no infrastructure for noticing trends with v3 style onions.

I also suspect that noticing trends is harder with v3-style onions, since each descriptor the hsdir sees is standalone, and it's not possible (without knowing the onion address) to link two descriptors to the same address.

So our only chance at estimating total number of v3 onion addresses is to know the publishing habits of v3 style onion services (how many descriptors per how much time period), and then publish the total number of descriptors we see, and folks can do some math afterwards to estimate number of running services? In any case we can see if the number goes up or down over time.

Or maybe there is some even better design? :)

The reason I bring it up now is (a) if we want to get any code into relays, we need to do it sufficiently before we need it, so it can get rolled out, and (b) I see discussions about bugs with v3-style onion services publishing every 2 minutes, and while we're fixing those we should keep in mind how handy it would be to be able to predict how many descriptors a new onion service will publish per time period on average.