Add metric to track the number of primary keys in valid doc id snapshot

[KKcorps]

This will help to detect any large inconsistencies b/w the number of primary keys in server vs the keys in snapshot.

The old PR got messy after rebase so raising a new one

[codecov-commenter]

Codecov Report

Merging #11515 (e25e744) into master (399f033) will increase coverage by 0.08%.
Report is 39 commits behind head on master.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master   #11515      +/-   ##
============================================
+ Coverage     62.99%   63.07%   +0.08%     
- Complexity     1094     1108      +14     
============================================
  Files          2302     2320      +18     
  Lines        124025   124528     +503     
  Branches      18901    19010     +109     
============================================
+ Hits          78126    78548     +422     
- Misses        40351    40373      +22     
- Partials       5548     5607      +59     

Flag	Coverage Δ
integration	<0.01% <0.00%> (-0.01%)	⬇️
integration1	<0.01% <0.00%> (-0.01%)	⬇️
integration2	0.00% <0.00%> (ø)
java-11	63.03% <33.33%> (+0.07%)	⬆️
java-17	62.93% <33.33%> (+0.08%)	⬆️
java-20	62.89% <33.33%> (+0.03%)	⬆️
temurin	63.07% <33.33%> (+0.08%)	⬆️
unittests	63.07% <33.33%> (+0.08%)	⬆️
unittests1	67.51% <33.33%> (-0.02%)	⬇️
unittests2	14.49% <0.00%> (+0.01%)	⬆️

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Files Changed	Coverage Δ
...cal/upsert/BasePartitionUpsertMetadataManager.java	39.61% <0.00%> (-0.45%)	⬇️
...a/org/apache/pinot/common/metrics/ServerGauge.java	91.42% <100.00%> (+0.25%)	⬆️

... and 159 files with indirect coverage changes

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