IssueTracker

We have been using traffic migration (e.g. `gcloud app services set-traffic --migrate`) for a while now to move traffic to newly deployed versions.  However, we've ended up finding we want even more control over how fast the traffic moves over, to allow us to start out very slowly while caches are still being warmed up, so we've moved to setting a different traffic split every 30 seconds to move traffic over a few percent at a time.  Sadly, that means we have to give up traffic migration, which means we tend to see some request aborted errors from the scaler, if we move the traffic over a little faster than it would like.

We'd really love to not have to choose!  That is, we'd love to be able to use `--migrate` when moving from one multi-version split to another.  This would help us minimize the impact on our site from moving traffic to a new version.

To be explicit, we would like to be able to run (assuming we start with all traffic on v1):
gcloud app deploy --version v2
gcloud app services set-traffic --migrate v1=95,v2=5
gcloud app services set-traffic --migrate v1=90,v2=10  # a minute later
gcloud app services set-traffic --migrate v1=85,v2=15  # another minute later
and so on.  Right now, this fails with error messages like "ERROR: (gcloud.app.services.set-traffic) The migrate flag can only be used with splits to a single version."  (The equivalent option in the Cloud Console UI is also disabled.)