Emit some stats from executor

Emit the load average, a counter for builds, and a guage for
running builds.

Change-Id: I8541724f1322b8257b623b3b2cfd8f3e6b95574d
diff --git a/tests/unit/test_scheduler.py b/tests/unit/test_scheduler.py
index c1f0a0e..ab0d0fd 100755
--- a/tests/unit/test_scheduler.py
+++ b/tests/unit/test_scheduler.py
@@ -117,6 +117,8 @@
         self.assertReportedStat(
             'zuul.tenant.tenant-one.pipeline.gate.project.review_example_com.'
             'org_project.master.total_changes', value='1|c')
+        exec_key = 'zuul.executor.%s' % self.executor_server.hostname
+        self.assertReportedStat(exec_key + '.builds', value='1|c')
 
         for build in self.history:
             self.assertTrue(build.parameters['zuul']['voting'])