[FLINK-38704][metrics] Fix MetricConfig.getString() to handle numeric…

[mukul-8]

Backport of #27743

What is the purpose of the change

Fixes the Prometheus metrics reporter (and potentially other reporters) not loading configured port values.

The issue occurs because numeric YAML configuration values are stored as Integer/Long objects, but Properties.getProperty() only returns Strings. When MetricConfig.getString() is called for numeric values like metrics.reporter.prom.port, it returns null, causing reporters to fall back to default values.

Note: The bug only reproduces when a single number is assigned to the port (e.g., 9999). It works correctly when a port-range is used (e.g., 9000-9100) because ranges are stored as Strings.

Workaround: Quote the port value in YAML configuration: metrics.reporter.prom.port: "9999"

Brief change log

• Changed MetricConfig.getString() to use get() instead of getProperty(), matching the pattern of getInteger(), getLong(), etc. Added test cases to verify the fix.

Verifying this change

This change fixes the bug where:

• Configuration loads: metrics.reporter.prom.port=9999
• But reporter starts on default port 9249 instead

After the fix, the configured port is correctly applied.

Added test case to verify this change is working fine.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 47c7ab5 Azure: SUCCESS

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