fix(tiering): implement qlist symmetric cooloff and fix memory handling

[romange]

Implements the symmetric cooloff logic for QList tiering that walks from both ends of the list and offloads nodes beyond the threshold. Fixes memory handling bugs in move operator and null pointer check.

Important: everything is done in dry-run - no actual tiering happens.

Changes:

• Implement symmetric cooloff in CoolOff() for balanced tiering
• Fix InsertNode() null pointer check when insert_opt is AFTER
• Fix move operator to properly reset num_offloaded_nodes_
• Add Stats::operator+= for cross-shard aggregation
• Add Prometheus metrics for list reads and tiering events
• Add list_tiering_threshold flag for configuration

[Copilot]

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[augmentcode]

🤖 Augment PR Summary

Summary: Improves list tiering support in QList and extends observability around list access/tiering.

Changes:

• Implements symmetric CoolOff() traversal to offload interior list nodes from both ends beyond the configured depth threshold
• Fixes move-assignment state handling for num_offloaded_nodes_ and hardens InsertNode() ID calculation when inserting AFTER with a null old_node
• Adds QList::Stats::operator+= and aggregates per-thread qlist stats into server metrics
• Exports new Prometheus counters for list reads and tiered list offload/onload events
• Adds --list_tiering_threshold flag to enable/configure list tiering at promotion time

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[augmentcode]

Review completed. 2 suggestions posted.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.