Poll for lag stat in IngestFromKafkaIT

[andrross]

The stat is populated asynchronously, so poll for the expected value.

Example failure: https://build.ci.opensearch.org/job/gradle-check/84759/testReport/org.opensearch.plugin.kafka/IngestFromKafkaIT/testAllActiveOffsetBasedLag/

Check List

• Functionality includes testing.

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[github-actions]

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🧪 PR contains tests

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[github-actions]

✅ Gradle check result for e4b1c74: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.73%. Comparing base (28c01db) to head (e4b1c74).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##               main   #23019    +/-   ##
==========================================
  Coverage     71.73%   71.73%            
- Complexity    77567    77600    +33     
==========================================
  Files          6168     6168            
  Lines        360105   360105            
  Branches      52380    52380            
==========================================
+ Hits         258307   258315     +8     
- Misses        81173    81270    +97     
+ Partials      20625    20520   -105     

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