[FLINK-22528][docs] Document latency tracking metrics for state accesses

[Myasuka]

What is the purpose of the change

Add document of latency tracking metrics for state accesses.

Brief change log

Add document of latency tracking metrics for state accesses.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

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[tillrohrmann]

Thanks for creating this documentation PR @Myasuka. Has this PR already been reviewed by @carp84?

[Myasuka]

@rkhachatryan , would you please take a look at this PR?

[rkhachatryan]

Thanks for the PR @Myasuka

I've made some suggestions, PTAL.

[tillrohrmann]

Maybe @infoverload you also wanna take a look at this documentation PR.

[infoverload]

Suggested change

	Flink also allows to track the keyed state access latency for all state-backends. This feature is disabled by default.
	Flink also allows tracking the keyed state access latency for all state backends. This feature is disabled by default.

same for English version

[Myasuka]

I think this should be okay and this also existed in many places within Flink's docs.

[infoverload]

Suggested change

	Flink allows to track the latency of records travelling through the system. This feature is disabled by default.
	Flink allows tracking the latency of records travelling through the system. This feature is disabled by default.

same for English version

[Myasuka]

I think this should be okay and this also existed in many places within Flink's docs.

[rkhachatryan]

Thanks for updating the PR, I don't have any further concerns.

[Myasuka]

Thanks for @rkhachatryan 's review, merging...