code: public Object set(String key, Object value) {
          int idx = keys.indexOf(key);
          if (idx == -1) {
              keys.add(key);
              values.add(value);
              return null;
          }
          return values.set(idx, value);
      }
docs: set metadata `key` with `value` and returns the previous value
  This method won't remove if value == null