Add an ability to specify metric for unsafe routes (#474)

examples/config.yml

@@ -165,10 +165,13 @@ tun:
   # Unsafe routes allows you to route traffic over nebula to non-nebula nodes
   # Unsafe routes should be avoided unless you have hosts/services that cannot run nebula
   # NOTE: The nebula certificate of the "via" node *MUST* have the "route" defined as a subnet in its certificate
+  # `mtu` will default to tun mtu if this option is not specified
+  # `metric` will default to 0 if this option is not specified
   unsafe_routes:
     #- route: 172.16.1.0/24
     #  via: 192.168.100.99
-    #  mtu: 1300 #mtu will default to tun mtu if this option is not sepcified
+    #  mtu: 1300
+    #  metric: 100
 
 
 # TODO