In reference to the discussion in #168

The field should probably be set as the number of concurrent streams (governed by --max-active-requests), multiplied by the number specified in concurrency. However, --concurrency can be set to auto in the CLI, making it difficult for the output transform code to read it.

The output transform code shouldn't use the number of CPUs on the machine when concurrency == 'auto' because it may run on a different machine.

As a prerequisite: We need to offer an "effective concurrency" field in the NH output proto. Then the output transform code will not need to infer the concurrency.