report_stages: Add important as build metric field.

BUG=chromium:635968
TEST=./run_tests

Change-Id: I33450741ec0eab831ab84c927666661ac887351a
Previous-Reviewed-on: https://chromium-review.googlesource.com/372978
(cherry picked from commit 07ef04df0c0b46394f5ae89e7821e72cb0e333f2)
Reviewed-on: https://chromium-review.googlesource.com/410380
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/cbuildbot/stages/report_stages.py b/cbuildbot/stages/report_stages.py
index 91ba3a5..c2171d4 100644
--- a/cbuildbot/stages/report_stages.py
+++ b/cbuildbot/stages/report_stages.py
@@ -874,7 +874,8 @@
       duration = self._GetBuildDuration()
 
       mon_fields = {'status': status_for_db,
-                    'build_config': self._run.config.name}
+                    'build_config': self._run.config.name,
+                    'important': self._run.config.important}
       metrics.Counter(constants.MON_BUILD_COMP_COUNT).increment(
           fields=mon_fields)
       metrics.SecondsDistribution(constants.MON_BUILD_DURATION).add(