cross-namespace traffic graph

Nobody has claimed this yet.

## Feature Request

### What problem are you trying to solve?
I am only able to see a graph of traffic for a single namespace. In my cluster traffic will cross the namespace boundary so with Linkerd I will not be able to get a view of the full flow of traffic. I would like to be able to see the traffic for the whole cluster or a set of selected namespaces.

### How should the problem be solved?
Add a "namespace selection list" and then display a graph with telemetry data for all services/apps in those namespaces

### Any alternatives you've considered?
While it is possible to see cross-namespace requests for a selected service, I have not been able to find any way of view it at an overall level.

### How would users interact with this feature?
There would probably have to be a new page/view for just the graph that has a namespace selection list and displays the whole graph. From there a user could drill down to a specifc pod/deployment/service/.... or maybe even drill down to the namespace level

Kiali (used by Istio) provides this capability, so there may be something there to start with.

No implementation files or tests are named. Start by locating the existing single-namespace traffic graph and its telemetry/query entry point, then compare Kiali’s cross-namespace graph behavior. Done means a namespace selection list can render and support a whole-cluster or selected-namespace graph with drill-down.

Written by the indexing model from the issue text.