Keys can be pretty-printed using the Comparer.Format facility. We should add something similar for values. This is particularly useful for CRDB post-mortem debugging where values have rich structure. For CRDB, decoding of the value is dependent on the key, so the interface here will need to take both a key and value. Something like:

type PrettyValue func(key, value []byte) fmt.Formatter

For CRDB this would look something like:

PrettyValue = func(key, value []byte) string {
  decoded, err := storage.DecodeMVCCKey(key)
  if err != nil {
    return err.Error()
  }
  return kvserver.SprintKeyValue(storage.MVCCKeyValue{Key: decoded, Value: value}, false)
}

Open question is how to associate this PrettyValue function with the comparer. Should RegisterComparer take an optional PrettyValue?