Sharing a pool of gRPC threads/connections with N topics for Pub/Sub?

**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.

Pub/Sub Publisher のスレッドセーフティに関するリファレンスと、issue で言及されている MakePublisherConnection の実装から始めてください。1 つのスレッドプールを複数の topic 間で共有できるかどうかを確認し、そのうえでサンプルまたは焦点を絞ったガイダンスを使って想定される使用方法を文書化してください。複数 topic の接続とスレッドプールの動作が明確になれば完了です。

索引モデルが issue の本文から書いたものです。