[Pull-based Ingestion] Fix lag metric when streaming source is empty

[varunbharadwaj]

Description

Lag is computed as the difference between system time and last processed message timestamp. In the case no message is consumed for a long time, the lag keeps increasing. We solve this by setting lag to 0 if empty batch is read from the streaming source. We also explicitly set lag to 0 if the poller is paused.

Related Issues

Follow up for #17077

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[github-actions]

✅ Gradle check result for 5da5636: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.87%. Comparing base (a132bb5) to head (5da5636).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...rch/indices/pollingingest/DefaultStreamPoller.java	75.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19393      +/-   ##
============================================
+ Coverage     72.81%   72.87%   +0.05%     
- Complexity    69838    69857      +19     
============================================
  Files          5674     5674              
  Lines        320850   320890      +40     
  Branches      46383    46390       +7     
============================================
+ Hits         233624   233843     +219     
+ Misses        68361    68073     -288     
- Partials      18865    18974     +109     

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