nixos/systemd-networkd: Allow three value variant in ipv6AcceptRAConfig.RouteMetric option

nixos/lib/systemd-lib.nix

@@ -278,6 +278,21 @@ rec {
       attr ? ${name} && !(hasPrefix "@" attr.${name})
     ) "Systemd ${group} field `${name}' is not a systemd credential";
 
+  assertRouteMetricOrTriple =
+    name: group: attr:
+    let
+      isMetric = n: 0 <= n && 4294967295 >= n;
+
+      parts = splitString ":" attr.${name};
+      partsValid =
+        length parts == 3 && all (p: (match "[0-9]+" p) != null && isMetric (toIntBase10 p)) parts;
+      valid = (isInt attr.${name} && isMetric attr.${name}) || partsValid;
+    in
+    optional (attr ? ${name} && !valid) (
+      "Systemd ${group} field `${name}' must either be an integer in the range [0,4294967295]"
+      + " or a string containing three integers separated with `:`"
+    );
+
   checkUnitConfig =
     group: checks: attrs:
     let

nixos/modules/system/boot/networkd.nix

@@ -1744,8 +1744,7 @@ let
         (assertValueOneOf "UseDNR" boolValues)
         (assertValueOneOf "UseDomains" (boolValues ++ [ "route" ]))
         (assertRange "RouteTable" 0 4294967295)
-        (assertInt "RouteMetric")
-        (assertRange "RouteMetric" 0 4294967295)
+        (assertRouteMetricOrTriple "RouteMetric")
         (assertValueOneOf "QuickAck" boolValues)
         (assertValueOneOf "UseMTU" boolValues)
         (assertValueOneOf "UseHopLimit" boolValues)