Benchmark scenario "dotnet_protobuf_async_unary_ping_pong_1MB" is highly flaky

Also see internal bug b/253283712.

`dotnet_protobuf_async_unary_ping_pong_1MB` is run with grpc_e2e_performance_gke] on both 30-core machine and 8-core machines

Since https://source.cloud.google.com/results/invocations/38c6078b-4413-488b-81a9-22a71226dcc8/targets (the job was broken before) there are multiple failures of this test.

It failed typically with driver process hang and the loadtest eventually timed out.

Start with the grpc_e2e_performance_gke configuration and the dotnet_protobuf_async_unary_ping_pong_1MB scenario, then review the linked performance results. Reproduce the driver-process hang or loadtest timeout on the 30-core and 8-core machines, and verify that repeated runs no longer fail or time out.

Written by the indexing model from the issue text.