INT-4037: Graph: Add Link Type To LinkNode

JIRA: https://jira.spring.io/browse/INT-4037

Provide type information in `LinkNode`s, input, output, error, discard.

This might be rendered as:

```
              +---(discard)
              |
         +----o----+
         |         |
         |         |
         |         |
(input)--o         o---(output)
         |         |
         |         |
         |         |
         +----o----+
              |
              +---(error)
```

  "links" : [ {
    "from" : 10,
    "to" : 9,
    "type" : "output"
  }, {
    "from" : 10,
    "to" : 1,
    "type" : "error"
  } ]

Document `link.type` in the `graph.adoc`

spring-integration-core/src/main/java/org/springframework/integration/support/management/graph/IntegrationGraphServer.java

@@ -205,7 +205,7 @@ public class IntegrationGraphServer implements ApplicationContextAware, Applicat
 			nodes.add(handlerNode);
 			MessageChannelNode channelNode = channelNodes.get(handlerNode.getInput());
 			if (channelNode != null) {
-				links.add(new LinkNode(channelNode.getNodeId(), handlerNode.getNodeId()));
+				links.add(new LinkNode(channelNode.getNodeId(), handlerNode.getNodeId(), LinkNode.Type.input));
 			}
 			producerLink(links, channelNodes, handlerNode);
 		}
@@ -217,19 +217,19 @@ public class IntegrationGraphServer implements ApplicationContextAware, Applicat
 		if (endpointNode.getOutput() != null) {
 			channelNode = channelNodes.get(endpointNode.getOutput());
 			if (channelNode != null) {
-				links.add(new LinkNode(endpointNode.getNodeId(), channelNode.getNodeId()));
+				links.add(new LinkNode(endpointNode.getNodeId(), channelNode.getNodeId(), LinkNode.Type.output));
 			}
 		}
 		if (endpointNode instanceof ErrorCapableNode) {
 			channelNode = channelNodes.get(((ErrorCapableNode) endpointNode).getErrors());
 			if (channelNode != null) {
-				links.add(new LinkNode(endpointNode.getNodeId(), channelNode.getNodeId()));
+				links.add(new LinkNode(endpointNode.getNodeId(), channelNode.getNodeId(), LinkNode.Type.error));
 			}
 		}
 		if (endpointNode instanceof DiscardingMessageHandlerNode) {
 			channelNode = channelNodes.get(((DiscardingMessageHandlerNode) endpointNode).getDiscards());
 			if (channelNode != null) {
-				links.add(new LinkNode(endpointNode.getNodeId(), channelNode.getNodeId()));
+				links.add(new LinkNode(endpointNode.getNodeId(), channelNode.getNodeId(), LinkNode.Type.discard));
 			}
 		}
 	}

spring-integration-core/src/main/java/org/springframework/integration/support/management/graph/LinkNode.java

@@ -29,9 +29,12 @@ public class LinkNode {
 
 	private final int to;
 
-	public LinkNode(int from, int to) {
+	private final Type type;
+
+	public LinkNode(int from, int to, Type type) {
 		this.from = from;
 		this.to = to;
+		this.type = type;
 	}
 
 	public int getFrom() {
@@ -42,4 +45,12 @@ public class LinkNode {
 		return this.to;
 	}
 
+	public Type getType() {
+		return this.type;
+	}
+
+	public enum Type {
+		input, output, error, discard
+	}
+
 }

src/reference/asciidoc/graph.adoc

@@ -61,7 +61,8 @@ A simple Spring Integration application with only the default components would e
   "links": [
     {
       "from": 2,
-      "to": 3
+      "to": 3,
+      "type": "input"
     }
   ]
 }
@@ -76,7 +77,32 @@ Other properties are provided by the framework and allows you to distinguish a s
 The `links` graph element represents connections between nodes from the `nodes` graph element and, therefore, between integration components in the source Spring Integration application.
 For example _from_ a `MessageChannel` _to_ an `EventDrivenConsumer` with some `MessageHandler`;
 or _from_ an `AbstractReplyProducingMessageHandler` _to_ a `MessageChannel`.
+For the convenience and to allow to determine a link purpose, the model is supplied with the `type` attribute.
+The possible types are:
+
+- _input_ - identify the direction from `MessageChannel` to the endpoint; `inputChannel` or `requestChannel` property;
+- _output_ - the direction from `MessageHandler`, `MessageProducer` or `SourcePollingChannelAdapter` to the `MessageChannel` via an `outputChannel` or `replyChannel` property;
+- _error_ - from `MessageHandler` on `PollingConsumer` or `MessageProducer` or `SourcePollingChannelAdapter` to the `MessageChannel` via an `errorChannel` property;
+- _discard_ - from `DiscardingMessageHandler` (e.g. `MessageFilter`) to the `MessageChannel` via `errorChannel` property.
+
 The information from this element can be used by a visualizing tool to render connections between nodes from the `nodes` graph element, where the `from` and `to` numbers represent the value from the `nodeId` property of the linked nodes.
+For example the link `type` can be used to determine the proper _port_ on the target node:
+
+----
+              +---(discard)
+              |
+         +----o----+
+         |         |
+         |         |
+         |         |
+(input)--o         o---(output)
+         |         |
+         |         |
+         |         |
+         +----o----+
+              |
+              +---(error)
+----
 
 The `nodes` graph element is perhaps the most interesting because its elements contain not only the runtime components with their `componentType` s and `name` s, but can also optionally contain metrics exposed by the component.
 To enable the metrics, add an `@EnableIntegrationManagement` to some `@Configuration` class or add an `<int:management/>` element to your XML configuration.