Support cross tenancy TraceQL queries

[braunsonm]

Is your feature request related to a problem? Please describe.
It is not uncommon to want to understand the parent/child relationship between spans that exist in different tenants. This is one of the major motivators of a tracing backend in the first place. The recent addition of cross tenancy queries is great but has a major limitation, particularly:

TraceQL queries that compare multiple spansets may not correctly return all traces in a cross-tenant query

This does hamper the usability of capturing traces across a large organization that need to have systems in different tenants but for administrators that need to understand the full request flow of a trace.

Describe the solution you'd like
Allow for TraceQL queries to span multiple tenants

Describe alternatives you've considered
There are no alternatives besides reporting all your spans in the same tenant. Which is not ideal for large organizations that have different quotas per tenant.

[mghildiy]

I would like to work on this one.

[joe-elliott]

This will be immensely difficult.

The only viable option would be to do query sharding in the frontend that requested subsets of data from each tenant and then finished query evaluation in the frontend w/ the per tenant data. It would be a major architectural change.