code: public ToStringHelper add(String name, Object value) {
      checkNotNull(name);
      addHolder(value).builder.append(name).append('=').append(value);
      return this;
    }
docs: Adds a name/value pair to the formatted output in {@code name=value}
  format. If {@code value} is {@code null}, the string {@code "null"}
  is used, unless {@link #omitNullValues()} is called, in which case this
  name/value pair will not be added.