swarming: log the number of active futures
swarming: log the number of active futures

This is for the log like
https://screenshot.googleplex.com/B2ds6aH7D4PRv6o

Bug: 1027431
Change-Id: I281ed81d7a704a9e401bd6d176e8fc2381e48ab1
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2672985
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>

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