Outgoing requests from the Wikimedia production environment must go through an HTTP proxy. We need to ensure that outgoing image labeling requests support this.
Looking under the hood of the Cloud Vision PHP client library, it's built around the Guzzle HTTP client library, which accepts a proxy option. If the library provides a reasonable way to configure a proxy option on our image annotation requests, then that's what we should do.
If this isn't reasonably possible, we may have to update the GoogleCloudVisionHandler to make its requests using MwHttpRequest implementations.