Add explicit annotation for Prometheus port to scrape

* Without the prometheus.io/port annotation, Prometheus
service discovery can scrape other Prometheus ports that
may be available.
* For example, Prometheus sidecars (not included) may
be scraped and that may be unintended

addons/prometheus/service.yaml

@@ -5,6 +5,7 @@ metadata:
   namespace: monitoring
   annotations:
     prometheus.io/scrape: 'true'
+    prometheus.io/port: '9090'
 spec:
   type: ClusterIP
   selector: