public synchronized void addNested(final MutableMetrics metrics) {
        if (finalized) throw new IllegalStateException("Metrics have been finalized and cannot be modified");
        this.nested.put(metrics.getId(), metrics);
    }
Create a {@code MutableMetrics} from an immutable one.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/MutableMetrics.java