dbconsole: include rebalancing.cpupersecond graph in the replication dashboard

[kvoli]

QPS is displayed per-node on the replication dashboard in DB Console. In #96127 we introduced CPU based rebalancing, where previously there was only QPS.

This issue is to introduce an additional graph in the replication metrics dashboard for the rebalancing.cpupersecond per node.

Jira issue: CRDB-25082

[blathers-crl]

Hi @kvoli, please add branch-* labels to identify which branch(es) this release-blocker affects.

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