@Override
        public void serializeWithType(final TraversalMetrics property, final JsonGenerator jsonGenerator,
                                      final SerializerProvider serializerProvider, final TypeSerializer typeSerializer) throws IOException {
            serializeInternal(property, jsonGenerator);
        }
Maps in the JVM can have {@link Object} as a key, but in JSON they must be a {@link String}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphson/GraphSONSerializersV1.java