swarming: log who calls cancel API
swarming: log who calls cancel API

Bug: 1091147
Change-Id: I86e38673cc3686ab6c81b1be75ccd7ff16a23504
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2232241
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py