The actual read times from the database are also tracked elsewhere (Instrumentation.java IIRC) as well. Can I work on this part in a different PR? I agree with you on your comments. 

As a side note, I really don't like that metrics_locator is the one that feeds the list of rollups to be computed. metrics_locator is populated by both raw metrics and pre-agg metrics. It would be nice if there is another metrics_locator_preagg. It might avoid the problem of creating a runnable only to realize no work needs to be done.