What component of google-cloud-cpp is this feature request for?
Pub/Sub

Is your feature request related to a problem? Please describe.
Thanks for a great product. I've been looking at your samples for using the Pub/Sub publisher class. There is one example in particular where it shows you how to pass in your own thread pool:
https://cloud.google.com/cpp/docs/reference/pubsub/latest/classgoogle_1_1cloud_1_1pubsub_1_1Publisher#thread-safety

I had some suspicions about gRPC connections and the like, so I dug into the source code.

Each call of MakePublisherConnection is still tied to a topic. It is unclear if can use the same thread pool for multiple calls to MakePublisherConnection or not. Inside MakePublisherConnection I see gRPC channels being connected per thread, so I'm assuming that it's not expected to use the same thread pool across multiple topics?

That leaves me to my point of confusion. What is the expected usage for a scenario that I would assume is common:

1. You have multiple topics you publish to
2. You want a pool of gRPC connections that can be shared across all topics

Describe the solution you'd like
I would like a sample to document that, or even have this issue be the documentation.

Describe alternatives you've considered
I am currently iterating on ways I can play with the API to do what I expect, but it isn't ideal.