bng.stats passed by default as topic to kpi exporter

Change-Id: I23125b595f80b411f4b1b72785c5dfe0c6864442
diff --git a/nem-monitoring/values.yaml b/nem-monitoring/values.yaml
index 0578f41..6039b6a 100644
--- a/nem-monitoring/values.yaml
+++ b/nem-monitoring/values.yaml
@@ -33,6 +33,7 @@
     - voltha.kpis
     - onos.aaa.stats.kpis
     - onos.kpis
+    - bng.stats
 
 prometheus:
   alertmanager: