[CELEBORN-1491][FOLLOWUP] Using baseLegend for metrics_FlushWorkingQueueSize_Value

### What changes were proposed in this pull request?

Followup for https://issues.apache.org/jira/browse/CELEBORN-1491, use baseLegend for the new introduced metrics.

### Why are the changes needed?

As title.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Before:
<img width="852" alt="image" src="https://github.com/user-attachments/assets/cf1cb852-9480-49ff-873c-62b535167fa3">

After:
<img width="346" alt="image" src="https://github.com/user-attachments/assets/cbd6ec82-4531-4056-b8ee-96bde813f899">

<img width="849" alt="image" src="https://github.com/user-attachments/assets/a787be53-4646-48d2-a24e-da9b714b7fca">

Closes #2712 from turboFei/grafana_dashboard.

Authored-by: Wang, Fei <fwang12@ebay.com>
Signed-off-by: mingji <fengmingxiao.fmx@alibaba-inc.com>

assets/grafana/celeborn-dashboard.json

@@ -2550,7 +2550,7 @@
               "fullMetaSearch": false,
               "includeNullMetadata": true,
               "instant": false,
-              "legendFormat": "__auto",
+              "legendFormat": "${baseLegend}",
               "range": true,
               "refId": "A",
               "useBackend": false