Activator in Istio mesh

[josephburnett]

/area autoscale
/kind dev

Expected Behavior

• The Activator component routes requests directly to Pod IP addresses when they become available after activation.

Actual Behavior

• When sidecar injection is enabled, routing to Pods directly is not supported.

Steps to Reproduce the Problem

1. Turn on Istio sidecar injection
2. Turn on the enable-scale-to-zero experiment
3. Deploy a revision and wait for it to scale to zero
4. Send a request to the revision and look at the Activator logs

Additional Info

[akyyy]

I have a draft, but there is some problem with it.
#833

In that draft, I had to clear the request header in activator.go to overcome 404's.
revRequest.r.Host = ""

According to Nghia, the right fix could be additional route rules or check with istio team. we should try those first.