emit compaction pause duration metric in every compaction batch

[miancheng7]

Previously, the compaction pause duration metric was skipped if the number of keys was smaller than batchNum, leading to gaps in monitoring data. This change ensures the metric is emitted for every batch, regardless of key count.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: miancheng7
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[k8s-ci-robot]

Hi @miancheng7. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[fuweid]

Hi @miancheng7 you need to sign it off by git commit -s --amend. thanks