Observability for Pub/Sub subscriber

The Subscriber client does a lot for applications under the surface. That is great, but sometimes surfacing errors can make it useful for applications to troubleshoot their environments. e.g. a flaky network may result in many messages being redelivered.

Our library should help applications diagnose issues and optimize performance by offering some sort of observability.

## Signals

Googlers can see go/pubsub-debug-logging for a list of valuable signals to collect.

We are not committing to all of them, but it will at least be useful to know
- when a stream is restarted under the hood
- when an ack/nack/modack RPC fails silently