decodeGroup = decoder => ({
  errorCode: decoder.readInt16(),
  groupId: decoder.readString(),
  state: decoder.readString(),
  protocolType: decoder.readString(),
  protocol: decoder.readString(),
  members: decoder.readArray(decoderMember),
})
DescribeGroups Response (Version: 1) => throttle_time_ms [groups]
  throttle_time_ms => INT32
  groups => error_code group_id state protocol_type protocol [members]
    error_code => INT16
    group_id => STRING
    state => STRING
    protocol_type => STRING
    protocol => STRING
    members => member_id client_id client_host member_metadata member_assignment
      member_id => STRING
      client_id => STRING
      client_host => STRING
      member_metadata => BYTES
      member_assignment => BYTES