[server] Add coordinator metrics for rebalance

[morazow]

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• I searched in the issues and found nothing similar.

Description

Motivation

Rebalance currently exposes no metrics at all. The only way to observe a rebalance is to poll Admin#listRebalanceProgress or tail coordinator logs. #3965 (PR #3985) improves the Admin API with timestamps and historical listing, but that covers on-demand queries, not continuous monitoring. Operators cannot:

• Alert when a rebalance is running, stuck, or failing.
• Chart migration throughput and per-bucket duration on a dashboard.
• Correlate a rebalance window with write-latency or disk-usage changes on TabletServers.

The coordinator already registers cluster gauges such as bucketCount through CoordinatorMetricGroup (fluss-server/.../metrics/group/CoordinatorMetricGroup.java), and RebalanceManager already tracks all the needed state (inProgressRebalanceTasksQueue, inProgressRebalanceTasks, finishedRebalanceTasks, the in-flight task start timestamp), so this is a wiring task, not a new subsystem.

Proposed solution

Register a rebalance metric set on the coordinator metric group, with names added to MetricNames:

Gauges for the current task:

• rebalanceInProgress (0 or 1)
• rebalanceBucketsPending
• rebalanceBucketsCompleted, rebalanceBucketsFailed, rebalanceBucketsTimedOut
  (split of the finished map by per-bucket status)
• rebalanceDurationMs (time since the task was registered, 0 when idle)
• inflightBucketDurationMs (time the current bucket task has been running, 0 when idle)

Counters across tasks (survive task completion, reset on coordinator failover):

• rebalancesCompletedTotal, rebalancesFailedTotal, rebalancesCanceledTotal

Implementation notes:

• Expose the values from RebalanceManager via supplier gauges. The backing maps are ConcurrentHashMap and the in-flight fields are volatile, so reads from the metrics reporter thread are safe without touching the coordinator event thread.
• Register on CoordinatorMetricGroup, following the existing bucketCount pattern.
• Document the new metrics in website/docs/maintenance/observability/monitor-metrics.md.

Out of scope: per-table or per-bucket metric groups (cardinality), and the Admin API side already covered by #3965.

Willingness to contribute

• I'm willing to submit a PR!