Related with #13344.
In the current implementation, it doesn't keep warming if SDS based secret entity can't be extracted because of the existence of runtime feature flag. envoy.reloadable_features.cluster_keep_warming_no_secret_entity.
To remove this, we should have delayed activate behavior on clusterInit. I'm considering implementing with this approach.

• If it failed to create secure transport socket factory after initial_fetch_timeout occurred, It doesn't change state from warming. When it was called, we should activate delayed callback which will activate the cluster by executing onClusterInit at regular intervals.

It is only proposal. So I'm glad to introduce other approaches. cc @htuch