public Builder addMetric(Metric metric) {
            if (metrics == null) {
                metrics = new ArrayList<>();
            }
            metrics.add(metric);
            return this;
        }
Contains all metrics within a family (that is, of the same name). All metrics in a family have the same type.
oap-server/server-library/library-util/src/main/java/org/apache/skywalking/oap/server/library/util/prometheus/metrics/MetricFamily.java