code: func AddLogStreamTagFilter(q *ds.Query, key string, value string) *ds.Query {
  	if value == "" {
  		return q.Eq("_Tags", encodeKey(key))
  	}
  	return q.Eq("_Tags", encodeKey(fmt.Sprintf("%s=%s", key, value)))
  }
docs: // AddLogStreamTagFilter adds a tag filter to a Query object.
  //
  // This method will only add equality filters to the query. If value is empty,
  // a presence filter will be added; otherwise, an equality filter will be added.
  //
  // This incorporates the encoding expressed by TagMap.