Add --namespace=prometheus to examples in metrics exporting guide

Author: deliahu

docs/clusters/observability/metrics.md

@@ -110,7 +110,7 @@ spec:
 Then, update Prometheus with your annotations:
 
 ```bash
-kubectl patch prometheuses.monitoring.coreos.com prometheus --patch-file patch.yaml --type merge
+kubectl patch --namespace prometheus prometheuses.monitoring.coreos.com prometheus --patch-file patch.yaml --type merge
 ```
 
 ## Long term metric storage
@@ -138,5 +138,5 @@ spec:
 Update Prometheus with your changes:
 
 ```bash
-kubectl patch prometheuses.monitoring.coreos.com prometheus --patch-file patch.yaml --type merge
+kubectl patch --namespace prometheus prometheuses.monitoring.coreos.com prometheus --patch-file patch.yaml --type merge
 ```