code: def format_key_from_backend(self, key):
          
          if not self.prefix:
              return key
          if not key.startswith(self.prefix):
              raise StorageError("Key {!r} does not start with expected prefix {!r}".format(key, self.prefix))
          return key[len(self.prefix):]
docs: Strip the configured prefix from a key retrieved from the backend
          before passing it on to other pghoard code and presenting it to the
          user.