Task list with rate limiting per key

There are multiple use cases that require rate limiting per dynamically created list of keys. For example some downstream API has rate limit per calling customer. This is possible to achieve using task list throttling, but as the set of keys is usually dynamic managing client side workers is a non trivial task.

The proposed feature is to provide a special task list type that would support rate limiting per some "partition key". So when an activity is scheduled the key should be provided when invoking it. And on the worker side no changes are needed (besides the mechanism to configure the task list itself).