Add Tempo / Tenants dashboard

[yvrhdn]

What this PR does:
Adds a new dashboard: Tempo / Tenants.
Example (with single tenant data unfortunately):

Features:

• first half contains cluster-level information, grouped by tenant. This is useful to get an overview of the cluster and the size of the tenants present. We always show the topk(10) tenants.
• the lower rows are all tenant-specific graphs allowing you to see how a specific tenant is doing.

I've also tweaked the namespace selector in Tempo Operational, this didn't work if you don't have the kube_pod_container_info metric. Using tempo_build_info is similar to all the other dashboards.

Which issue(s) this PR fixes:
Closes #813 (I think?)

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[github-actions]

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