This attribute indicates the sum of cost metrics of trunks that make
up the selected route returned by RouteSelector. If the cost metrics
at both ends of a trunk differ, the larger metric is used for that trunk.
If a route could not be selected for whatever reason then the
routeCostMetric is set to a value 999999 intended to symbolize
unknown cost. For manual routes the value of routeCostMetric is
also 999999. This is because RouteSelector does not verify manual
routes thus routeCostMetric is unknown.