Merge pull request #6158 from jtattermusch/performance_testing_polishing

src/csharp/Grpc.IntegrationTesting/ClientRunners.cs

src/node/performance/worker.js

src/node/performance/worker_service_impl.js

src/proto/grpc/testing/control.proto

test/cpp/qps/driver.cc

test/cpp/qps/driver.h

test/cpp/qps/qps_driver.cc

test/cpp/qps/qps_json_driver.cc

test/cpp/qps/report.cc

test/cpp/qps/report.h

test/cpp/util/benchmark_config.cc

tools/run_tests/performance/scenario_config.py

tools/run_tests/run_performance_tests.py

tools/run_tests/tests.json