[I] [Bug] CURRENT_FETCH_EVENT_TIME_LAG metric calculation is wrong when the datastream is completely blocked [paimon]

[via GitHub]

xiangyuf opened a new issue, #4244:
URL: https://github.com/apache/paimon/issues/4244

   ### Search before asking
   
   - [X] I searched in the [issues](https://github.com/apache/paimon/issues) 
and found nothing similar.
   
   
   ### Paimon version
   
   master(8ac121a)
   
   ### Compute Engine
   
   Flink
   
   ### Minimal reproduce step
   
   Check the CURRENT_FETCH_EVENT_TIME_LAG metric for a completely blocked 
datastream. It will not grow over time. But the CURRENT_EMIT_EVENT_TIME_LAG 
will continue to grow.
   
   ### What doesn't meet your expectations?
   
   The CURRENT_FETCH_EVENT_TIME_LAG metric should grow over time even if the 
datastream is completely blocked.
   
   ### Anything else?
   
   The lastSplitUpdateTime will not update for a completely blocked datastream.
   
![img_v3_02f1_22fc6ab6-2887-4dbd-94a6-fbb04603193g](https://github.com/user-attachments/assets/b5894dc8-d4cf-48df-9ebe-4baa0461e3b2)
   
   In flink, the CURRENT_EMIT_EVENT_TIME_LAG is calculated by idleTime or 
currentTime, so this metric can keep grow for a completely blocked datastream.
   
![img_v3_02f1_226f7c12-7838-4cac-8453-2dc9e367812g](https://github.com/user-attachments/assets/267f504d-753d-4cd6-ae59-1359f2b38b4e)
   
   
   ### Are you willing to submit a PR?
   
   - [X] I'm willing to submit a PR!


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