query all tenants at once

[phin1x]

Is your feature request related to a problem? Please describe.
cortex supports multiple tenants ids in the X-Scope-OrgID header (if the feature is activated). at the moment you have to specify a explicit list of tenant ids in the header but you can't query all tenants at once.

we are using cortex for our kubernetes clusters where every namespace is a customer. at the moment we are using a modified version of the prometheus auth project from rancher to provide multi tenancy but this is a bit hacky. a cleaner solution would be that every namespace becomes a tenant in cortex. as an administrator we want to query all tenants but this would add much more complexity because we need to compute a list of all namespaces from all clusters.

Describe the solution you'd like
allow a user to query all tenants, for example with a wildcard (single *) in the X-Scope-OrgID header or a second header in the http request (for example X-Scope-Administrator).

Describe alternatives you've considered

Additional context

[anjmao]

I also need something similar to display data from multiple tenants in single grafana dashboard. Maybe querier could be configured to understand multi tenant queries and add/remove tenantID labels. Also querier could allow to return labels with all known tenants for tenant dropdown variable eg:(label_values(cortex_tenant_info))

[stale]

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