cas: log kvs files
cas: log kvs files

This is to see the size of db files when download is slow.

Bug: 1172879
Change-Id: Id128efc5997f5542ee69f4fc9e2beca7b3064637
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2827445
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go