$ ./ip/ip r a 2.2.2.0/24 via 10.0.2.123 dev mgmt0 proto 188 metric 20
$ ./ip/ip r | grep 2.2.2
2.2.2.0/24 via 10.0.2.123 dev mgmt0 proto 188 metric 20
$ mkdir -p /etc/iproute2/rt_protos.d
$ echo "188 foo" > /etc/iproute2/rt_protos.d/bar.conf
$ ./ip/ip r | grep 2.2.2
2.2.2.0/24 via 10.0.2.123 dev mgmt0 proto foo metric 20
It works on my side:
$ ./ip/ip r a 2.2.2.0/24 via 10.0.2.123 dev mgmt0 proto 188 metric 20 rt_protos. d rt_protos. d/bar.conf
$ ./ip/ip r | grep 2.2.2
2.2.2.0/24 via 10.0.2.123 dev mgmt0 proto 188 metric 20
$ mkdir -p /etc/iproute2/
$ echo "188 foo" > /etc/iproute2/
$ ./ip/ip r | grep 2.2.2
2.2.2.0/24 via 10.0.2.123 dev mgmt0 proto foo metric 20