Fix high-watermark metric returning -1 for leader

[samratdotcom]

Summary

Fixes a bug where the high-watermark metric in KafkaRaftMetrics temporarily reports -1 when a Raft node transitions to the Leader state.

Detail

Previously, QuorumState.transitionToLeader initialized the new LeaderState with an empty high watermark. This caused the metric to drop to -1 until the first follower fetch updated it. This change updates QuorumState to pass the existing high watermark (from the previous Follower or Candidate state) to the LeaderState, preserving monotonicity.

Tests

• Updated: KafkaRaftMetricsTest to assert the correct high watermark value (e.g., 5) instead of -1.
• Updated: QuorumStateTest (testHighWatermarkRetained) to verify the high watermark is preserved on transition.
• Updated: LeaderStateTest to match the updated constructor signature.
• Verified that all tests pass with ./gradlew raft:test.

Committer Checklist (Excluded from Commit Message)

• Verify that tests pass.
• Verify that checkstyle passes.

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NOTE TO MAINTAINERS:
I am currently unable to create a JIRA ticket due to the public signup restrictions on the ASF JIRA portal. Could a maintainer please help create a ticket for this issue?

[chia7712]

@samratdotcom thanks for this patch. Would you mind removing the unrelated changes?

[samratdotcom]

Hi @chia7712 ,
Review feedback addressed: I've removed all unrelated whitespace and formatting changes. The diff now contains only the necessary logic for the high-watermark fix.