docs: 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);
    }