class_docstr: Use a function turn facet value into a label
  
  Parameters
  ----------
  func : callable
      Function to label an individual string
code: def __call__(
      self, label_info: strip_label_details
  ) -> strip_label_details:
      label_info = label_info.copy()
      variables = label_info.variables
      for facet_var, facet_value in variables.items():
          variables[facet_var] = self.func(facet_value)
      return label_info
  
imports: from __future__ import annotations
  import typing
  from abc import ABCMeta, abstractmethod
  from ..exceptions import PlotnineError
label_desc: Use image node_id 2 for calling the _function_labeller obj's underlying member method code with example usage: obj.__call__(label_info) and returns: label_info