[flink] Include lake reader in source metrics and correct max timestamp lag semantics

[zuston]

Purpose

Linked issue: close #3304

Brief change log

1. Add the lake reader into the source metrics, this is necessary for the long-term backfill when using the fluss multi-layer log table as the source
2. Correct the max timestamp log semantics on subscribing the multi buckets or partitions. that is mentioned in [flink] currentFetchEventTimeLag is misleading when subscribing multiple partitions/buckets with uneven lag #3304
3. Make these metrics collecting logic into one place in FlinkRecordsWithSplitIds

Tests

API and Format

Documentation

[zuston]

cc @swuferhong @luoyuxia