cbuildbot_launch: Add metrics for CleanupChroot stage.

Capture metrics for the CleanupChroot stage so we can see how often it
fails.

BUG=chromium:855151
TEST=run_tests
     cros tryjob --local success-build

Change-Id: Ia02a6e7a4ef9e0828bfa19477b47f7b7e0fd87c9
Reviewed-on: https://chromium-review.googlesource.com/1112713
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Mike Nichols <mikenichols@chromium.org>
diff --git a/scripts/cbuildbot_launch.py b/scripts/cbuildbot_launch.py
index 9c741f3..8c97a22 100644
--- a/scripts/cbuildbot_launch.py
+++ b/scripts/cbuildbot_launch.py
@@ -53,6 +53,7 @@
 METRIC_DISTFILES_CLEANUP = (
     'chromeos/chromite/cbuildbot_launch/distfiles_cleanup')
 METRIC_DEPOT_TOOLS = 'chromeos/chromite/cbuildbot_launch/depot_tools_prep'
+METRIC_CHROOT_CLEANUP = 'chromeos/chromite/cbuildbot_launch/chroot_cleanup'
 
 # Builder state
 BUILDER_STATE_FILENAME = '.cbuildbot_build_state.json'
@@ -502,7 +503,9 @@
           if result == 0 else constants.BUILDER_STATUS_FAILED)
       SetLastBuildState(root, build_state)
 
-      CleanupChroot(buildroot)
+      with metrics.SecondsTimer(METRIC_CHROOT_CLEANUP, fields=metrics_fields):
+        CleanupChroot(buildroot)
+
       return result