[gold] Add metrics for finding the closest digests
[gold] Add metrics for finding the closest digests

Change-Id: I223450e1ec94cf834ff5c660b90bedad366f82ac
Bug: skia:9080
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/273256
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot