release-23.2: ui: omit tenant selector in Metrics when only one exists

[blathers-crl]

Backport 1/1 commits from #119882 on behalf of @dhartunian.

/cc @cockroachdb/release

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The "Virtual Cluster" selector on the Metrics page should only be present when we have more than one tenant on the cluster. Previously, the code that decided this did not account for the fact that an additional "All" option was always present in the dropdown and mistook it for a tenant when counting.

Resolves: #114168
Epic: None

Release note: None

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Release justification: low risk high impact bugfix to reduce confusion in DB Console for standard deployments

[blathers-crl]

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