[KYUUBI apache#6839] Add example for service monitor in Helm chart

### Why are the changes needed?

Adding  an example to `servicemonitor` in values.yaml to improve the usability .

### How was this patch tested?

execute command `helm install kyuubi-test .`
check the servicemonitor after deployment

### Was this patch authored or co-authored using generative AI tooling?

No

Closes apache#6839 from zhifanggao/add_default_endpoints.

Closes apache#6839

59aac41 [Cheng Pan] Update charts/kyuubi/values.yaml
869f45e [zhifanggao] include the key itself

Lead-authored-by: Cheng Pan <[email protected]>
Co-authored-by: zhifanggao <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>

charts/kyuubi/values.yaml

@@ -289,6 +289,8 @@ metrics:
     enabled: false
     # List of service endpoints serving metrics to be scraped by Prometheus, see Prometheus Operator docs for more details
     endpoints: []
+    #  endpoints:
+    #    - port: prometheus
     # Additional labels to be used to make ServiceMonitor discovered by Prometheus
     labels: {}