zoekt: surface dashboard to show difference between repos assigned v. tracked

Created by: ggilmore

Context:

[zoekt-indexserver's queue] isn't a normal queue (ie a double linked list), it really is more of a simple scheduler. It is a priority queue, but it keeps track of repos which has already been popped off the queue. This is so it can assign a higher priority if a repo gets back in the queue if the commit needed to index changes / the last index failed / etc. As the help describes, this should be the same size as the number of repos assigned to the host. So seeing this value is more useful for confirming there aren't bugs. Maybe a less prominent dashboard showing the difference between num assigned and this metric?

I could create a new row for this dashboard and hide it by default, but maybe it's worth showing more prominently if there if this is already showing a bug in my local instance