the monitor could display the number of unload requests outstanding in the tserver
That would be cool. I could see the general premise being otherwise useful too.
Perhaps related, does tablet unload interrupt running scans? Or, does a scan have the ability to block unloads indefinitely? Perhaps the tserver should try for some amount of time to unload, if it still hasn't unloaded because a scan is running, forcefully abort it? That also begs the question in the case of custom iterators, can we make something that will gracefully abort a scan using such iterators or are we reliant on users implementing exception handling properly to avoid the "9 day query"?