Modify clients to consistently send N% of initial consensus requests to the authorities

[teor]

We currently use the directory authority statistics to estimate the number of clients in the Tor network.

But fallback directories will handle initial consensus downloads in future releases.

So, to preserve client statistics, we can modify Tor clients to make a certain small percentage (1%?) of initial consensus downloads to the directory authorities. If we scale the authority downloads from each Tor release by this factor, we can estimate the number of clients.

This is particularly important for client IPv6 bootstrap (#17840), as only some authorities and fallbacks have IPv6 (and the proportion of fallbacks is lower than the proportion of authorities).

We could change the meaning of DirAuthorityFallbackRate to "the authorities see this proportion of queries", or implement another option, and document the interaction with DirAuthorityFallbackRate.

(I think the second option is better, but we'd end up essentially obsoleting DirAuthorityFallbackRate.)

[teor]

Currently, the authorities report serving about 2MB/s worth of consensuses, or about 1-2 consensuses per second.

Once default fallbacks are release, we can calculate the proportion of client requests that we expect to go to each authority using DirAuthorityFallbackRate / (SUM(fallback.weight) + COUNT(authorities)).

If all 413 fallback directories in [0] were accepted, this would be:
1 / (6585231 + 9) per authority, or 9 / 6585240 in total.

The authorities would see about 3-4 consensus downloads per month under this new scheme, which means that we'd have to rely on relay statistics to count clients.

Alternately, we could make this change and use it for stats.

Is the change a good idea by itself?
Does it get us anything?

[0]: https://trac.torproject.org/projects/tor/attachment/ticket/15775/fallback_dirs.inc

[teor]

I don't think we need to fix anything in tor here, the existing client counts already use relay statistics, and fallback directories report statistics as relays.

But we may need to adjust how we analyse the data slightly.

​https://metrics.torproject.org/clients-data.html