generic_stages: Add build important/experimental as stage metric field.

BUG=chromium:635958
TEST=./run_tests

Change-Id: Id6800ee4f8e2655b35bc46a55a48e9688c4cd1b7
Previous-Reviewed-on: https://chromium-review.googlesource.com/383954
(cherry picked from commit efa85ed1d0c67691fb10e7ee9ce37f31bfb889cc)
Reviewed-on: https://chromium-review.googlesource.com/407860
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/generic_stages.py b/cbuildbot/stages/generic_stages.py
index aaec44b..8c2ca50 100644
--- a/cbuildbot/stages/generic_stages.py
+++ b/cbuildbot/stages/generic_stages.py
@@ -206,7 +206,8 @@
 
     fields = {'status': status,
               'name': self.name,
-              'build_config': self._run.config.name}
+              'build_config': self._run.config.name,
+              'important': self._run.config.important}
 
     metrics.SecondsDistribution(constants.MON_STAGE_DURATION).add(
         elapsed_time_seconds, fields=fields)