If a given delta response contains too many resources, the server will break it
up into multiple responses. However, this means the client does not know whether
it received all the resources for its subscription. This is especially relevant
for wildcard subscriptions, for which the client does not know the resources
ahead of time and therefore cannot wait for them explicitly. By returning
additional metadata in the nonce (there is no field for this in the delta
discovery response, though I'm hoping that will change
cncf/xds#99), the client can know if the server
chunked the response, and react accordingly.