docs: def serialize_dict_keys(d, prefix=""):
      """returns all the keys in nested a dictionary.
      >>> sorted(serialize_dict_keys({"a": {"b": {"c": 1, "b": 2} } }))
      ['a', 'a.b', 'a.b.b', 'a.b.c']
      """
      keys = []
      for k, v in d.items():
          fqk = '{}{}'.format(prefix, k)
          keys.append(fqk)
          if isinstance(v, dict):
              keys.extend(serialize_dict_keys(v, prefix="{}.".format(fqk)))
  
      return keys