Make worker block lock pool size observable #14803

[elega]

What changes are proposed in this pull request?

Please outline the changes and how this PR fixes the issue.

Why are the changes needed?

Please clarify why the changes are needed. For instance,

1. If you propose a new API, clarify the use case for a new API.
2. If you fix a bug, describe the bug.

Does this PR introduce any user facing changes?

Please list the user-facing changes introduced by your change, including

1. change in user-facing APIs
2. addition or removal of property keys
3. webui

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[alluxio-bot]

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