Added live slice count

[ycchenzheng]

Description

Adding active_slices to the stepping log to help monitor elastic status

FIXES: b/520489064

Tests

Tested with different TPU versions & topologies

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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