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.

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Signed-off-by: Alexandru Gheorghe <[email protected]>
Co-authored-by: ordian <[email protected]>