code: public MetricName tagged(Map<String, String> add) {
          final Map<String, String> tags = new HashMap<>(add);
          tags.putAll(this.tags);
          return new MetricName(key, tags);
      }
docs: Add tags to a metric name and return the newly created MetricName.
  
  @param add Tags to add.
  @return A newly created metric name with the specified tags associated with it.