[autotest] Scheduler, drone_manager, drone_utility stats.

Adds some useful stats to drone manager, handle agents and
drone utility. These stats should help us track processes,
figure out where the drone_manager latency is coming from
and draw correlations between the number of agents scheduled
and drone refresh time.

This cl also moves site_drone_utility's kill_process method
into drone_utility, and modifies the nuke_pids function to only
wait on and kill processes that haven't already died.

TEST=Ran suites.

Change-Id: I56e6ee05fa2ae1935435dbc2055d7f99a9a89e5e
Reviewed-on: https://chromium-review.googlesource.com/211769
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Prashanth B <beeps@chromium.org>
Tested-by: Prashanth B <beeps@chromium.org>
9 files changed