We have some metrics around number of tasks completed over time. I think that we could use a couple of others:

1. Amount of time spent in each prefix. I think that we already store this data in all_durations (I brought this up when @ntabris did his work before as probably what we actually wanted, rather than number of tasks completed).
2. Amount of memory currently used by each prefix. This is probably not a counter, but instead an instantaneous measurement of current state

This would be useful for after-the-fact debugging of clusters. @dchudz was curious about this. @fjetter is this easy for someone on your team to do?