Split Redis information in structured logging by instance

Our structured logs contain two fields with information about our use of Redis: redis_calls and redis_duration_s.

We actually have multiple Redis instances. One request might spend all its time in the persistent Redis; another might spend its time in the cache Redis; and a third might split its team between the two. Right now, we can't tell.

We should add separate fields for these: redis_{cache,persistent,etc.}_{calls,duration_s}, so that we can track this separately. We can also keep the existing totals.