Candidate Phase Metric Collector Data Type

Updated to match new atom, this handles the candidate phase. Note this
is *per provider* and thus we store this in a collection until the
logging occurs. How that happens may slightly change given our new
funnel points. In any case, this local collection of the metric will
help us emit properly.

Bug: 270403549
Test: To be chained in - are visually tested
Change-Id: Iad387c33f10c43b31308b18c339d72f9176a658e