ObjectMapper mapper = new ObjectMapper();
JsonNode rootNode = mapper.readTree(m);

JsonNode metric_node = rootNode.path("metrics");
Iterator iterator  = metric_node.getFields();
while (iterator.hasNext()) {
     logger.info("class_name=> {}", iterator.next().getClass().getName());
     logger.info("content=> {}", iterator.next());
}


OUTPUT
class_name=> java.util.LinkedHashMap$Entry
content=> user.recent_category.read.success=0