code: def system(session):
      
      system_test_path = os.path.join("tests", "system.py")
      system_test_folder_path = os.path.join("tests", "system")
      # Sanity check: Only run tests if the environment variable is set.
      if not os.environ.get("GOOGLE_APPLICATION_CREDENTIALS", ""):
          session.skip("Credentials must be set via environment variable")
  
      system_test_exists = os.path.exists(system_test_path)
      system_test_folder_exists = os.path.exists(system_test_folder_path)
      # Sanity check: only run tests if found.
      if not system_test_exists and not system_test_folder_exists:
          session.skip("System tests were not found")
  
      # Use pre-release gRPC for system tests.
      session.install("--pre", "grpcio")
  
      # Install all test dependencies, then install this package into the
      # virtualenv's dist-packages.
      session.install("mock", "pytest")
      for local_dep in LOCAL_DEPS:
          session.install("-e", local_dep)
      session.install("-e", "../test_utils/")
      session.install("-e", ".")
  
      # Run py.test against the system tests.
      if system_test_exists:
          session.run("py.test", "--quiet", system_test_path, *session.posargs)
      if system_test_folder_exists:
          session.run("py.test", "--quiet", system_test_folder_path, *session.posargs)
docs: Run the system test suite.