Implement metrics to measure snowflake churn

As discussed in the meeting this week, it would be useful to know how often snowflake proxy IP addresses actually change. We collect counts of unique IPs on any given day, but not how much variance we get in IP addresses over time.

This relates to our ability to resist censorship, as snowflake relies in part on the claim that snowflakes are ephemeral, changing, and difficult to block exhaustively.

Let's implement some metrics to see how much snowflake IPs change.