@Override
    public MutableMetrics getNested(final String metricsId) {
        return (MutableMetrics) nested.get(metricsId);
    }
Set an annotation value. Support exists for Strings and Numbers only. During a merge, Strings are concatenated
into a "," (comma) separated list of distinct values (duplicates are ignored), and Numbers are summed.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/MutableMetrics.java