Add metric to measure the time it takes to gather enough assignments (#4587)

To understand with high granularity how many assignment tranches are
triggered before we concur that we have enough assignments.

This metric is important because the triggering of an assignment creates
a lot of work in the system for approving the candidate and gossiping
the necessary messages.

---------

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
Co-authored-by: ordian <write@reusable.software>