Add completion timestamp and historical listing to RebalanceProgress

[affo]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Admin#listRebalanceProgress returns RebalanceProgress, which carries rebalanceId, status, a progress fraction, and a per-bucket map — but no timestamp for when a rebalance started or finished. In addition, calling listRebalanceProgress(null) only ever returns the currently in-progress rebalance task; there's no way to retrieve information about rebalances that have already completed.

This makes it impossible to build any operator-facing tooling that answers "when did the cluster last rebalance?" or "how long did the last rebalance take?" — useful for health dashboards, alerting, and audit trails. Today that information is only recoverable by tailing server logs.

Solution

• Add startedAt/completedAt (or equivalent) timestamp fields to RebalanceProgress.
• Retain a bounded history of recently-completed rebalance tasks (server-side or ZooKeeper-backed) so that listRebalanceProgress (or a new method) can return past tasks, not just the current in-progress one — including their final status (COMPLETED/FAILED/CANCELLED) and timestamps.
• A retention window/limit (e.g. keep the last N completed tasks) would bound the added storage/memory cost.

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!

[morazow]

Hi @affo , I am going to take this. Please assign it to me 🤝