Add index metric for created active partitions

[lava]

Add a new index metric for created active partitions, so it becomes possible to monitor how many new partitions are created over time.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

[dominiklohmann]

This is nice. I question the value of one metric (personally I'd just drop it), and the other two require documentation on https://docs.tenzir.com/vast/features/metrics/

Thanks for this! 📈