[Flink] Add sourceScalingMaxParallelism metric for auto-scaling systems

[wzhero1]

Purpose

This PR adds a new metric sourceScalingMaxParallelism to the Flink Source Enumerator. This metric provides a recommended upper bound of parallelism for auto-scaling systems to optimize resource allocation.

Motivation

Auto-scaling systems need to understand the optimal parallelism for Paimon sources to:

1. Avoid over-provisioning resources for fixed-bucket tables (where parallelism shouldn't exceed bucket count)
2. Make informed scaling decisions for dynamic-bucket tables

The metric value:

• For fixed bucket tables: equals the bucket number
• For dynamic bucket tables (bucket = -1): equals the current parallelism

Note: This is a recommendation, not a hard limit - users can still configure higher parallelism manually if needed.

Tests

Added unit tests in FileStoreSourceMetricsTest.java:

• continuousFileStoreFixBucketEnumeratorMetricsTest() - Verifies metric equals bucket number for fixed bucket tables
• continuousFileStoreDynBucketEnumeratorMetricsTest() - Verifies metric equals current parallelism for dynamic bucket tables

Also added TestingMetricUtils.getGauge() helper method for testing Gauge metrics.

API and Format

No API or storage format changes. This only adds a new metric.

Documentation

Updated docs/content/maintenance/metrics.md with the new metric description.