sa: refactor how metrics and logging are set up

[jsha]

In #6982 we refactored db.WrappedMap a bit so InitWrappedDB and SetSQLDebug need to go through an accessor function to get at the underlying *borp.DbMap or *sql.DB. This is a bit awkward. We should arrange things such that either (a) we do the necessary setup before wrapping the deeper layers in a WrappedMap; or the relevant setup should be through a function in db so it can just directly access those unexported members.