code: public EvalPayload eval(String group, long timestamp, List<TagsValuePair> vs) {
      List<Subscription> subs = subscriptions.getOrDefault(group, Collections.emptyList());
      List<EvalPayload.Metric> metrics = new ArrayList<>();
      for (Subscription s : subs) {
        DataExpr expr = s.dataExpr();
        for (TagsValuePair pair : expr.eval(vs)) {
          EvalPayload.Metric m = new EvalPayload.Metric(s.getId(), pair.tags(), pair.value());
          metrics.add(m);
        }
      }
      return new EvalPayload(timestamp, metrics);
    }
docs: Evaluate expressions for all subscriptions associated with the specified group using
  the provided data.
  
  @param group
  Name of the group. At Netflix this is typically the cluster that includes
  the instance reporting data.
  @param timestamp
  Timestamp to use for the payload response.
  @param vs
  Set of values received for the group for the current time period.
  @return
  Payload that can be encoded and sent to Atlas streaming evaluation cluster.