Feature request: Expose per-country bridge stats

[mpalmer]

It would be helpful for bridge operators to see longer-term trends in the per-country usage patterns to assess the likelihood of having been blocked. This data is available using GETINFO status/clients-seen, but that doesn't appear to be supported in stem at present (although I have submitted an enhancement request). The CLIENTS_SEEN event is supported, but it isn't particularly scrape-time friendly.

Sadly, my Python skills are insufficient to even consider taking this on, so I won't be able to provide a PR.