BQ Write: Endurance/throughput benchmarking

This issue tracks benchmarking for the Bigquery Storage write client.

We want a benchmark with knobs for:
- num gRPC channels
- num writers
- num tables
- rows per batch
- bytes per row
- multiplex options (enabled, stream pool size, max outstanding requests)

The purpose of the benchmark is to get us some hard numbers for the client's performance, and also to catch bugs against production.

We also want to validate the performance numbers and eradicate any transient errors that slip through the cracks. We can also instrument our code to log transient errors so we can see our retry policies in action.