docs: def init_config(self, app):
          """Initialize config."""
          app.config.setdefault(
              'LOGGING_FS_LEVEL',
              'DEBUG' if app.debug else 'WARNING'
          )
          for k in dir(config):
              if k.startswith('LOGGING_FS'):
                  app.config.setdefault(k, getattr(config, k))
  
          # Support injecting instance path and/or sys.prefix
          if app.config['LOGGING_FS_LOGFILE'] is not None:
              app.config['LOGGING_FS_LOGFILE'] = \
                  app.config['LOGGING_FS_LOGFILE'].format(
                      instance_path=app.instance_path,
                      sys_prefix=sys.prefix,
                  )