Add a sampler for the `#stat` method of a connection pool, and record
those stats in Prometheus. This applies to all stats except
`checkout_timeout`, which is really more of a configuration item. It
also applies to all database connections we use: the main one, the Geo
tracking database, and any connection pools for database load balancing
hosts.

This requires a fix to Geo::TrackingBase.connected?, which wasn't
correctly reporting the connection's status.