rt_metrics
rtm.rtm_rmx = rt_metrics;
rt_metrics.rmx_mtu = strtonum(value, 0, UINT_MAX, &errstr);
rt_metrics.rmx_expire = relative_expire ?
rt_metrics.rmx_locks |= flag;
struct rt_metrics rt_metrics;
struct rt_metrics rtm_rmx; /* metrics themselves */
void rtm_setmetrics(u_long, const struct rt_metrics *,
struct rt_metrics *);
rtm_setmetrics(u_long which, const struct rt_metrics *in,
rtm_getmetrics(const struct rtentry *rt, struct rt_metrics *out)