Report initial pipeline changes under correct key

The normal pipeline gauge uses this key, but the initial gauge
was not yet updated.

Change-Id: Icc150eae0afe15e54bd9f3cddbe534ec2555dc10
diff --git a/tests/unit/test_scheduler.py b/tests/unit/test_scheduler.py
index 75b88fb..8023ef6 100755
--- a/tests/unit/test_scheduler.py
+++ b/tests/unit/test_scheduler.py
@@ -135,9 +135,11 @@
 
     def test_initial_pipeline_gauges(self):
         "Test that each pipeline reported its length on start"
-        self.assertReportedStat('zuul.pipeline.gate.current_changes',
+        self.assertReportedStat('zuul.tenant.tenant-one.pipeline.gate.'
+                                'current_changes',
                                 value='0|g')
-        self.assertReportedStat('zuul.pipeline.check.current_changes',
+        self.assertReportedStat('zuul.tenant.tenant-one.pipeline.check.'
+                                'current_changes',
                                 value='0|g')
 
     def test_job_branch(self):
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index 62ad4fa..f3cb479 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -679,8 +679,10 @@
             try:
                 for pipeline in tenant.layout.pipelines.values():
                     items = len(pipeline.getAllItems())
-                    # stats.gauges.zuul.pipeline.NAME.current_changes
-                    key = 'zuul.pipeline.%s' % pipeline.name
+                    # stats.gauges.zuul.tenant.<tenant>.pipeline.
+                    #    <pipeline>.current_changes
+                    key = 'zuul.tenant.%s.pipeline.%s' % (
+                        tenant.name, pipeline.name)
                     self.statsd.gauge(key + '.current_changes', items)
             except Exception:
                 self.log.exception("Exception reporting initial "