Backport of Add CE version of Gateway Upstream Disambiguation into release/1.15.x (#19862)

Add CE version of Gateway Upstream Disambiguation

Co-authored-by: Thomas Eckert <teckert@hashicorp.com>

.changelog/19860.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+connect: Solves an issue where two upstream services with the same name in different namespaces were not getting routed to correctly by API Gateways.
+```

agent/consul/discoverychain/gateway.go

@@ -229,7 +229,7 @@ func targetForResolverNode(nodeName string, chains []*structs.CompiledDiscoveryC
 	splitterName := splitterPrefix + strings.TrimPrefix(nodeName, resolverPrefix)
 
 	for _, c := range chains {
-		targetChainPrefix := resolverPrefix + c.ServiceName + "."
+		targetChainPrefix := resolverPrefix + c.ID()
 		if strings.HasPrefix(nodeName, targetChainPrefix) && len(c.Nodes) == 1 {
 			// we have a virtual resolver that just maps to another resolver, return
 			// the given node name

agent/consul/discoverychain/gateway_test.go

@@ -615,6 +615,7 @@ func TestGatewayChainSynthesizer_Synthesize(t *testing.T) {
 			chain: &structs.CompiledDiscoveryChain{
 				ServiceName: "foo",
 				Namespace:   "default",
+				Partition:   "default",
 				Datacenter:  "dc1",
 				StartNode:   "resolver:foo-2.default.default.dc2",
 				Nodes: map[string]*structs.DiscoveryGraphNode{