docs: def output_of(*cmd: Optional[str], **kwargs) -> str:
      """Invokes a subprocess and returns its output as a string.
  
      Args:
          cmd: Components of the command to execute, e.g. ["echo", "dog"].
          **kwargs: Extra arguments for asyncio.create_subprocess_shell, such as
              a cwd (current working directory) argument.
  
      Returns:
          A (captured output, captured error output, return code) triplet. The
          captured outputs will be None if the out or err parameters were not set
          to an instance of TeeCapture.
  
      Raises:
           subprocess.CalledProcessError: The process returned a non-zero error
              code and raise_on_fail was set.
      """
      result = cast(str, run_cmd(*cmd,
                                 log_run_to_stderr=False,
                                 out=TeeCapture(),
                                 **kwargs).out)
  
      # Strip final newline.
      if result.endswith('\n'):
          result = result[:-1]
  
      return result