Test with variable network conditions

[emcfarlane]

Is your feature request related to a problem? Please describe.

Tests rely on a network harness to roundtrip flows from the client to the server. Depending on the network conditions tests can display different behaviour or fail in surprising ways. Recently an in memory transport was added to resolve flaky cases that occurred between HTTP1/2. This helped find errors by reducing roundtrip latency in testing. However, we might find other issues by increasing latency but have no easy way to apply this flow conditioning.

Describe the solution you'd like

Enhance the in memory transport to provide different flow properties. Two important ones to add would be throughput limits and latency. We could then apply to specific tests cases or as a global network configuration to run all testing under different conditions.

Additional context
As a reference see grpc-go/latency.