Log total size read from and written to Redis

In our request logs (and our Sidekiq logs), we should log some additional fields:

1. The total size (in bytes) of data we read from the each Redis instance (see also #313 (closed); we don't need to combine the issues, just be aware of them).
2. The same, but that we wrote to the each Redis instance.

From #318 (comment 335891517):

1. Instrument request/response sizes in the Redis client, not in the Rails cache
2. Keep it simple and add up the sizes of the strings going to and from Redis, don't bother parsing too much
3. Consider making our instrumentation a bit more generic so that it also catches pipelined calls