pubsub: list subscription configs, not just names

[mgabeler-lee-6rs]

Is your feature request related to a problem? Please describe.
I have a use case for listing subscriptions where I want to check information in the SubscriptionConfig object. The underlying gRPC ListSubscriptions call includes this information (or at least the portion I need -- I'm not entirely sure if it includes everything needed to form a SubscriptionConfig?), but the way it is exposed via the Subscriptions API on pubsub.Client and the resulting iterator it returns discards all this information (https://github.com/googleapis/google-cloud-go/blob/master/pubsub/subscription.go#L90-L96), and requires a second gRPC call for each subscription to re-acquire it!

Describe the solution you'd like
A couple possible solutions present themselves:

1. A ListSubscriptionConfigs API on pubsub.Client that returns an iterator exposing SubscriptionConfigs instead of Subscriptions
2. A NextConfig() method on SubscriptionIterator

Describe alternatives you've considered
The workaround of calling Config() on every Subscription returned works, but is inefficient

[hongalex]

Hey, thanks for reporting! This is quite strange behavior indeed, and even though you already noted the workaround, I think this could be more user friendly.

Just opened a PR to address this.