Attributes is appropriate for plumbing optional objects, especially useful for a long plumbing path where components in the middle may not care or see all objects in the container. It's not the case for the params on newNewResolver(). Both the default port and the proxy detector are guaranteed to be there and the plumbing path is very short. In this case, a first-class object is more appropriate and easier to use.
The Helper will also have getSynchronizationContext() (#2649) and a method to parse and validate service config. We we also considering merging Listener into the Helper, to make NameResolver match the LoadBalancer API.
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