make use of 'metadataMap.instanceId' consistent

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/ribbon/eureka/DomainExtractingServerList.java

@@ -105,7 +105,7 @@ class DomainExtractingServer extends DiscoveryEnabledServer {
 			DiscoveryEnabledServer enabled = (DiscoveryEnabledServer) server;
 			InstanceInfo instance = enabled.getInstanceInfo();
 			if (instance.getMetadata().containsKey("instanceId")) {
-				return instance.getMetadata().get("instanceId");
+				return instance.getHostName()+":"+instance.getMetadata().get("instanceId");
 			}
 		}
 		return super.getId();

spring-cloud-netflix-core/src/test/java/org/springframework/cloud/netflix/ribbon/eureka/DomainExtractingServerListTests.java

@@ -98,7 +98,7 @@ public class DomainExtractingServerListTests {
 				actualServer instanceof DomainExtractingServer);
 		DomainExtractingServer des = DomainExtractingServer.class.cast(actualServer);
 		assertEquals("zone was wrong", zone, des.getZone());
-		assertEquals("instanceId was wrong", INSTANCE_ID, des.getId());
+		assertEquals("instanceId was wrong", HOST_NAME + ":" + INSTANCE_ID, des.getId());
 		return des;
 	}