code: @JsonIgnore
      public void setTag(String key, String value) {
          requireArgument(key != null && !key.trim().isEmpty(), "Tag key cannot be null.");
          requireArgument(!ReservedField.isReservedField(key), "Tag is a reserved tag name.");
          if (value == null || value.isEmpty()) {
              _tags.remove(key);
          } else {
              _tags.put(key, value);
          }
      }
docs: Sets a single tag for the query.
  
  @param  key    The key for the tag. Cannot null, empty or a reserved field name.
  @param  value  The tag value. If the value is null, the tag is removed from the query tags.