bson_iter_t knows the key length of the current element.

It computes the key length in _bson_iter_next_internal

   /* iterate from start to end of NULL-terminated key string */
   for (o = iter->off + 1; o < len; o++) {
      if (!data[o]) {
         iter->d1 = ++o;
         goto fill_data_fields;
      }
   }

And it does not change d1. So iter.d1 - iter.key (plus one?) should be the length of the key.