fix: helm chart serviceMonitor port definition

Current `ServiceMonitor` template defines `http` as destination port for
scraping metrics data, which is not set in the `Service` manifest,
however `diag` is.

This commit updates the `ServiceMonitor` manifest to use `diag` port,
which is exposed when prometheus is enabled.

Signed-off-by: Jernej Porenta <jernej.porenta@3fs.si>

charts/opa-kube-mgmt/templates/servicemonitor.yaml

@@ -18,7 +18,7 @@ metadata:
   {{- end }}
 spec:
   endpoints:
-  - port: http
+  - port: diag
     interval: {{ .Values.serviceMonitor.interval }}
   jobLabel: {{ template "opa.fullname" . }}
   namespaceSelector: