docs: clearly document that metric analysis is unsupported

[wparr-circle]

Per openkruise/rollouts#181, openkruise currently does not support metric analysis during rollouts. Adding a small piece of documentation in the comparison table to clarify this while the linked issue remains unresolved.

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Welcome @wparr-circle! It looks like this is your first PR to openkruise/openkruise.io 🎉

[kruise-bot]

@wparr-circle: PR needs rebase.

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