async offsetFetch({ groupId, topics }) {
    const offsetFetch = this.lookupRequest(apiKeys.OffsetFetch, requests.OffsetFetch)
    return await this[PRIVATE.SEND_REQUEST](offsetFetch({ groupId, topics }))
  }
@public
@param {object} request
@param {string} request.groupId
@param {object} request.topics - If the topic array is null fetch offsets for all topics. e.g:
                 [
                   {
                     topic: 'topic-name',
                     partitions: [
                       { partition: 0 }
                     ]
                   }
                 ]
@returns {Promise}