code: public void setTag(String tagName, String tagValue) {
      if (tagValue == null) {
        tagValue = "";
      }
      tagTable.put(tagName, tagValue);
    }
docs: Sets the named tag to the specified value.
  
  @param tagName name of the tag
  @param tagValue new value of the tag
  @throws MetricsException if the tagName conflicts with the configuration