[lake/tiering] Add pendingRecordLag metric for lake tiering

[beryllw]

Purpose

Linked issue: close #3464

Brief change log

Tests

API and Format

Documentation

[Copilot]

Pull request overview

This PR adds a new backlog-aware lake-tiering metric (pendingRecordsLag) to better represent “how long the oldest committed-but-not-tiered record has been waiting”, avoiding false positives caused by idle periods.

Changes:

• Introduces pendingRecordsLag metric name and documents it in the metrics guide.
• Adds estimatedPendingStartTimeMs tracking and new lag calculation helpers in LogTablet.
• Registers the new gauge in Replica and adds tests covering idle-then-write and caught-up reset scenarios.

Reviewed changes

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

Show a summary per file

File	Description
website/docs/maintenance/observability/monitor-metrics.md	Documents the new pendingRecordsLag gauge under table_bucket_lakeTiering.
fluss-server/src/test/java/org/apache/fluss/server/replica/ReplicaTest.java	Adds a test ensuring pendingRecordsLag starts from 0 after an idle period when new backlog appears.
fluss-server/src/test/java/org/apache/fluss/server/replica/NotifyReplicaLakeTableOffsetTest.java	Adds a test ensuring pendingRecordsLag is cleared/reset when tiering catches up.
fluss-server/src/main/java/org/apache/fluss/server/replica/Replica.java	Registers the new LAKE_PENDING_RECORDS_LAG gauge and refactors timestampLag gauge to use LogTablet#getTimestampLag.
fluss-server/src/main/java/org/apache/fluss/server/log/LogTablet.java	Implements backlog detection + best-effort pending-start estimate and exposes getPendingRecordsLag / getTimestampLag.
fluss-common/src/main/java/org/apache/fluss/metrics/MetricNames.java	Adds LAKE_PENDING_RECORDS_LAG metric name constant.

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[luoyuxia]

@beryllw Thanks for the pr. Left some comments. PTAL