[FLINK-38057][runtime] Fix the error value between getting and using idleStartTime in source reader metric group

[ruanhang1993]

What is the purpose of the change

This pull request fixed the error value between getting and using idleStartTime in source reader metric group.

Brief change log

Use a snapshot value when accessing the idleStartTime.

Verifying this change

This change is already covered by existing tests.

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

[flinkbot]

CI report:

• fa27a368b7663f39ae6df2de13480976274403a9 Azure: FAILURE

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