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

https://crrev.com/c/2232241 was for batch cancel request.
This is for single cancel request.

Bug: 1091147
Change-Id: I3e3ce94f610ed6a4cce949841df704c26eb4b9a6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2235394
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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