[client] measure time in file_path.rmtree()
[client] measure time in file_path.rmtree()

file_path.rmtree does the following steps.
- make_tree_deleteable
- fs.rmtree (shutil.rmtree) max_retry=3
- kill child processed
- fs.rmtree (shutil.rmtree)

This CL measures time for each action.

Bug: 1164137
Change-Id: Iffe9adb0736a3763d81832cccea833af8b9c9800
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2628639
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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