Emits lag metric when caught up

[noob-se7en]

When Partition Consumer is caught up, Server does not emit any lag metric.
This fails some alert promQL expressions. The metric must be zero for such cases.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.46%. Comparing base (59551e4) to head (d957743).
Report is 1750 commits behind head on master.

Files with missing lines	Patch %	Lines
...e/data/manager/realtime/IngestionDelayTracker.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #15102      +/-   ##
============================================
+ Coverage     61.75%   63.46%   +1.71%     
- Complexity      207     1481    +1274     
============================================
  Files          2436     2745     +309     
  Lines        133233   154310   +21077     
  Branches      20636    23806    +3170     
============================================
+ Hits          82274    97936   +15662     
- Misses        44911    48984    +4073     
- Partials       6048     7390    +1342     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (+99.99%)	⬆️
integration	100.00% <ø> (+99.99%)	⬆️
integration1	100.00% <ø> (+99.99%)	⬆️
integration2	0.00% <ø> (ø)
java-11	63.39% <50.00%> (+1.69%)	⬆️
java-21	63.36% <50.00%> (+1.73%)	⬆️
skip-bytebuffers-false	63.45% <50.00%> (+1.71%)	⬆️
skip-bytebuffers-true	63.29% <50.00%> (+35.56%)	⬆️
temurin	63.46% <50.00%> (+1.71%)	⬆️
unittests	63.46% <50.00%> (+1.71%)	⬆️
unittests1	56.03% <50.00%> (+9.14%)	⬆️
unittests2	33.99% <0.00%> (+6.26%)	⬆️

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[gortiz]

It looks good to me, but we made several mistakes in the past, so I would let @KKcorps or @Jackie-Jiang take the last call.