Track consensus fetch times per country?

Right now Tor directories track consensus download times, and put that in their extrainfo descriptors.

It would be useful to have this info broken down per country, so we can e.g. learn when vanilla Tor starts getting throttled in a particular country (like it did a few years ago). And hey, for obfs4 bridges, it would be neat to compare if they are able to deliver the consensus quickly when vanilla Tor isn't.

I don't think we should put per-country performance stats into extra-info descs here, mostly since they'd get too big. I also briefly considered saying "well I only care about these two countries, so let's put only those in the extrainfo descs", but I think that's not a great strategy either.

So I think the current goal for this ticket is to produce an easy patch for relay operators to use if they want to measure this data, and also to either make this patch sufficiently safe that we're ok if relay operators want to apply it -- or convince ourselves that it is too unsafe and it should not be used.