Add a subscriber throughput benchmark

[dbolduc]

We should add a throughput benchmark for the subscriber client, similar to https://cloud.google.com/blog/products/data-analytics/testing-cloud-pubsub-clients-to-maximize-streaming-performance

The benchmark should pull messages from a subscription and ack them, across N tasks.

(At least) the following should be configurable:

• test iterations
• test duration
• num clients
• num tasks
• num worker threads
• outstanding messages / bytes
• ack/nack flush duration

The benchmark should report

• the ack count for each iteration
• some sort of error rate
• (stretch) deadline expirations / message redeliveries

We need to run this thing for a while (>1 day) and report the results here.

The target throughput is around 600 MB/s on a 16 vCPU VM. I am not sure about the target error rate / redelivery rate.

---

I have a rough sketch of such a benchmark: https://github.com/dbolduc/google-cloud-rust/tree/1b01c325ca05f5ded4d60f0948d0cff63993ab02