[ruler] Allow Federated rules

[rdooley]

Currently ruler rules are tied to a single tenant, but there are use cases where it is useful to evaluate a rule against multiple tenants.

Rather than write individual rules against each tenant, it would be useful to be able to create rules that span multiple tenants similar to the query front end's ability to query multiple tenants.
What would the preferred way be to make this configurable?

[rdooley]

I took an initial pass at this in https://github.com/rdooley/cortex/tree/multitenant-ruler building off some work @andrejbranch did in master...andrejbranch:querier-multi-meta and would love some feedback

[bboreham]

Note we have always described Cortex' ability to handle separate data for each tenant as "multitenant"; we used the word "federated" when talking about querying more than one at a time.

[rdooley]

Updated the title to align with expected nomenclature, ty @bboreham

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[rdooley]

There is some ongoing discussion on the proposal, and this feature is still desired