code: function addGroup(state, action) {
    const groups = state.metadata ? Object.assign({}, state.metadata[GROUPS_KEY]) : {};
    groups[action.id] = action.config;
    const metadata = {};
    metadata[GROUPS_KEY] = groups;
    return Object.assign({}, state, {
      metadata: Object.assign({}, state.metadata, metadata),
    });
  }
docs: Add a group to the map's metadata section.
  @param {Object} state The redux state.
  @param {Object} action The action object.
  
  @returns {Object} The new state object.