[FLINK-26420][Connector] use numRecordsSendCounter

[JingGe]

What is the purpose of the change

This is the follow-up PR of #18825. We found that the new sink v2 interface will have a wrong numRecordsOut metric for the sink writers. We send a fixed number of records to the source, but the numRecordsOut of the sink continues to increase by the time.

Brief change log

• replace the records out counter with the records send counter in FileWriter
• replace the records and byte out counter with the records and byte send counter in AsyncSinkWriter
• migrate FileWriterTest and AsyncSinkWriterTest to AssertJ

Verifying this change

This change is already covered by existing tests, such as FileWriterTest and AsyncSinkWriterTest.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)

[flinkbot]

CI report:

• d77dc2c UNKNOWN
• 9d0f8c2 Azure: SUCCESS

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[JingGe]

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[JingGe]

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[AHeise]

LGTM. Also thank you for migrating the tests!