def optional_target_names
      optional_targets.keys.select { |key| key.to_s.start_with?("subscribing_to_") }.map { |key| key.slice(15..-1) }
    end
def optional_target_names
      optional_targets.keys.select { |key| key.to_s.start_with?("subscribing_to_") }.map { |key| key.slice(15..-1) }
    end
Returns optional_target names of the subscription from optional_targets field.
 @return [Array<Symbol>] Array of optional target names