Activator: Pass the revision namespace and name by appending http request headers.

[akyyy]

/area autoscale
/kind dev

Expected Behavior

When we use activator, the istio routerules we created should provide all information so activator would know the target revision for the request. For example,

		   route:
		   - destination:
		       name: autoscale-configuration-00001-service
		       namespace: default
		  - destination:
		       name: activator-service
		       namespace: ela-system
		    appendHeaders:
		      Elafros-Revision: autoscale-configuration-00001
		    weight: 2
		   - destination:
		       name: autoscale-configuration-00002-service
		       namespace: default
		    weight: 98
		  - destination:
		       name: activator-service
		       namespace: ela-system
		    appendHeaders:
		      Elafros-Revision: autoscale-configuration-00002

Actual Behavior

Since appendHeaders is a field for routerule spec instead of destination,
https://github.com/istio/api/blob/78da6e6eb4ad4f158fb58e02f94efde4abf4cabf/routing/v1alpha1/route_rule.proto#L158
we do not have granularity to append different revision name for activator-service destination.

Additional Info

[akyyy]

This allows us support revisions in non-default namespace.

[akyyy]

https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/istio-dev/7l1zTx9zcc4/wwMGMFGbBgAJ
This needs an istio feature, which should be in v1alpha3.

[akyyy]

Deal with the case when the route has multiple traffic targets, specifically, when the length of route.Status.Traffic > 1.

[josephburnett]

@akyyy, what part of this is still incomplete? Just namespace?