cache: introduce trace around AddFileWithoutValidation
cache: introduce trace around AddFileWithoutValidation

This is for https://crbug.com/1172879#c73

This shows trace like
https://screenshot.googleplex.com/3Vm89NpR7AsfUR2
https://screenshot.googleplex.com/AtFdBtAyzLX2wmf
https://screenshot.googleplex.com/5JsnHynHZZmU6jY

I'll upload trace to cas in run_isolated.py if it takes time to
download.

Bug: 1172879
Change-Id: I6dcc689ccce7d58a802bc577598b58c6b65e7ce9
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2807223
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@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