[lake/tiering] add tiering table pending time and freshness metrics

[beryllw]

Purpose

Linked issue: close #3075

Brief change log

Add two new table-level metrics for lake tiering monitoring:

• pendingTime: Tracks how long a table has been waiting in the pending queue
• freshness: Exposes the user-configured table data freshness interval

Tests

API and Format

Documentation

[Copilot]

Pull request overview

Adds two new lake tiering table-level metrics to improve observability of tiering behavior and configuration, and documents them in the website metrics reference.

Changes:

• Add pendingTime gauge to expose how long a table has been waiting in the tiering pending queue.
• Add freshness gauge to expose the configured table data freshness interval (ms).
• Update metrics documentation to include the two new metrics and adjust table rowspans accordingly.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
website/docs/maintenance/observability/monitor-metrics.md	Documents the new lakeTiering_table metrics and updates table rowspans.
fluss-server/src/main/java/org/apache/fluss/server/coordinator/LakeTableTieringManager.java	Implements the new per-table gauges and tracks pending-entry timestamps.
fluss-common/src/main/java/org/apache/fluss/metrics/MetricNames.java	Adds metric name constants for pendingTime and freshness.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.