[KYUUBI #6840] Fix PodMonitor pods selection

### Why are the changes needed?

I am using the podminitor of kyuubi metrics, But I found using the podmonitor's selector.matchLabels can not select correct pods . So I fix it .

I also changed the values.yaml to add an example of podmonitor to improve the usability.

### How was this patch tested?

```
helm install kyuubi .
# kubectl get podmonitor kyuubi -oyaml
apiVersion: monitoring.coreos.com/v1
kind: PodMonitor
metadata:
  annotations:
    meta.helm.sh/release-name: kyuubi
    meta.helm.sh/release-namespace: default
  creationTimestamp: "2024-12-06T07:46:12Z"
  generation: 1
  labels:
    app.kubernetes.io/instance: kyuubi
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/name: kyuubi
    app.kubernetes.io/version: 1.8.0
    helm.sh/chart: kyuubi-0.1.0
  name: kyuubi
  namespace: default
  resourceVersion: "7583800"
  uid: 0dd11e43-d126-434e-988a-7f1914586e2b
spec:
  podMetricsEndpoints:
  - path: /metrics
    port: prometheus
  selector:
    matchLabels:
      app.kubernetes.io/instance: kyuubi
      app.kubernetes.io/name: kyuubi
# kubectl get po -l app.kubernetes.io/instance=kyuubi
NAME       READY   STATUS    RESTARTS   AGE
kyuubi-0   1/1     Running   0          24m
kyuubi-1   1/1     Running   0          22m
# kubectl get po -l  app.kubernetes.io/name=kyuubi
NAME       READY   STATUS    RESTARTS   AGE
kyuubi-0   1/1     Running   0          24m
kyuubi-1   1/1     Running   0          23m
```
### Was this patch authored or co-authored using generative AI tooling?

No

Closes #6840 from zhifanggao/change_podmonitor.

Closes #6840

092f13f [zhifanggao] change the podmonitor to select pods correctly

Authored-by: zhifanggao <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>
(cherry picked from commit 99e27d4)
Signed-off-by: Cheng Pan <[email protected]>

Author: zhifanggao

charts/kyuubi/templates/kyuubi-podmonitor.yaml

@@ -25,7 +25,7 @@ metadata:
 spec:
   selector:
     matchLabels:
-      app: {{ .Release.Name }}
+      {{- include "kyuubi.selectorLabels" . | nindent 6 }}
   podMetricsEndpoints:
     {{- toYaml .Values.metrics.podMonitor.podMetricsEndpoints | nindent 4 }}
 {{- end }}

charts/kyuubi/values.yaml

@@ -288,6 +288,9 @@ metrics:
     enabled: false
     # List of pod endpoints serving metrics to be scraped by Prometheus, see Prometheus Operator docs for more details
     podMetricsEndpoints: []
+    #  podMetricsEndpoints：
+    #    - path: /metrics
+    #      port: prometheus
 
   # ServiceMonitor by Prometheus Operator
   serviceMonitor: