docs: def fill(metrics_headers=()):
      """Add the metrics headers known to GAX.
  
      Return an OrderedDict with all of the metrics headers provided to
      this function, as well as the metrics known to GAX (such as its own
      version, the GRPC version, etc.).
      """
      # Create an ordered dictionary with the Python version, which
      # should go first.
      answer = collections.OrderedDict((
          ('gl-python', platform.python_version()),
      ))
  
      # Add anything that already appears in the passed metrics headers,
      # in order.
      for key, value in collections.OrderedDict(metrics_headers).items():
          answer[key] = value
  
      # Add the GAX and GRPC headers to our metrics.
      # These come after what may have been passed in (generally the GAPIC
      # library).
      answer['gax'] = gax.__version__
      # pylint: disable=no-member
      answer['grpc'] = pkg_resources.get_distribution('grpcio').version
      # pylint: enable=no-member
  
      return answer