[FLINK-16053][python] Remove redundant metrics in PyFlink

[dianfu]

What is the purpose of the change

We have recorded the metrics about how many elements it has processed in Python UDF. This kind of information is not necessary as there is also this kind of information in the Java operator. I have performed a simple test and find that removing it could improve the performance about 5% - 10%.

Brief change log

• Removes redundant metrics in PyFlink

Verifying this change

This change is a performance improvement work and have verified in my local environment.

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, Yarn/Mesos, 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)

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Last check on commit b48b577 (Fri Feb 14 07:50:41 UTC 2020)

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[HuangXingBo]

@dianfu Thanks a lot for the PR.
+1 from my side.

[dianfu]

@HuangXingBo Thanks a lot for the review. Merging...