code: public final void addExtraTag(final String name, final String value) {
      if (name.length() <= 0) {
        throw new IllegalArgumentException("empty tag name, value=" + value);
      } else if (value.length() <= 0) {
        throw new IllegalArgumentException("empty value, tag name=" + name);
      } else if (name.indexOf('=') != -1) {
        throw new IllegalArgumentException("tag name contains `=': " + name
                                           + " (value = " + value + ')');
      } else if (value.indexOf('=') != -1) {
        throw new IllegalArgumentException("tag value contains `=': " + value
                                           + " (name = " + name + ')');
      }
      if (extratags == null) {
        extratags = new HashMap<String, String>();
      }
      extratags.put(name, value);
    }
docs: Adds a tag to all the subsequent data points recorded.
  <p>
  All subsequent calls to one of the {@code record} methods will
  associate the tag given to this method with the data point.
  <p>
  This method can be called multiple times to associate multiple tags
  with all the subsequent data points.
  @param name The name of the tag.
  @param value The value of the tag.
  @throws IllegalArgumentException if the name or the value are empty
  or otherwise invalid.
  @see #clearExtraTag