[Metrics] Add tablet count on different status metrics

[acelyc111]

Based on these metrics, we can add alerts to remind
admins whether the cluster is healthy or not.

Proposed changes

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Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Code refactor (Modify the code structure, format the code, etc...)

Checklist

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• I have created an issue on (Fix #ISSUE) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• If these changes need document changes, I have updated the document
• Any dependent changes have been merged

Further comments

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