[FLINK-27105][docs] fix ChangelogStorage.uploadQueueSize metric type

[zoltar9264]

What is the purpose of the change

Fix metric type of ChangelogStorage.uploadQueueSize in document. see : FLINK-27105

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)

[flinkbot]

CI report:

• be5255d Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[rkhachatryan]

Thanks for the fix @zoltar9264 , LGTM

@flinkbot run azure

[rkhachatryan]

CI failure unrelated, build passed in my local branch, merging.