Add STOMP/WebSocket stats collection

This change adds collection of stats in key infrastructure components
of the WebSocket message broker config setup and exposes the gathered
information for logging and viewing (e.g. via JMX).

WebSocketMessageBrokerStats is a single class that assembles all
gathered information and by default logs it once every 15 minutes.
Application can also easily expose to JMX through an MBeanExporter.

A new section in the reference documentation provides a summary of
the available information.

Issue: SPR-11739

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompBrokerRelayMessageHandler.java

@@ -21,6 +21,7 @@ import java.util.Map;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
 
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -118,6 +119,8 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 	private final Map<String, StompConnectionHandler> connectionHandlers =
 			new ConcurrentHashMap<String, StompConnectionHandler>();
 
+	private final Stats stats = new Stats();
+
 
 	/**
 	 * Create a StompBrokerRelayMessageHandler instance with the given message channels
@@ -352,6 +355,13 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 		return this.headerInitializer;
 	}
 
+	/**
+	 * Return a String describing internal state and counters.
+	 */
+	public String getStatsInfo() {
+		return this.stats.toString();
+	}
+
 
 	@Override
 	protected void startInternal() {
@@ -380,6 +390,7 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 		SystemStompConnectionHandler handler = new SystemStompConnectionHandler(headers);
 		this.connectionHandlers.put(handler.getSessionId(), handler);
 
+		this.stats.incrementConnectCount();
 		this.tcpClient.connect(handler, new FixedIntervalReconnectStrategy(5000));
 	}
 
@@ -469,6 +480,7 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 			}
 			StompConnectionHandler handler = new StompConnectionHandler(sessionId, stompAccessor);
 			this.connectionHandlers.put(sessionId, handler);
+			this.stats.incrementConnectCount();
 			this.tcpClient.connect(handler);
 		}
 		else if (StompCommand.DISCONNECT.equals(command)) {
@@ -479,6 +491,7 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 				}
 				return;
 			}
+			stats.incrementDisconnectCount();
 			handler.forward(message, stompAccessor);
 		}
 		else {
@@ -615,6 +628,7 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 		 */
 		protected void afterStompConnected(StompHeaderAccessor connectedHeaders) {
 			this.isStompConnected = true;
+			stats.incrementConnectedCount();
 			initHeartbeats(connectedHeaders);
 		}
 
@@ -877,4 +891,33 @@ public class StompBrokerRelayMessageHandler extends AbstractBrokerMessageHandler
 		}
 	}
 
+	private class Stats {
+
+		private final AtomicInteger connect = new AtomicInteger();
+
+		private final AtomicInteger connected = new AtomicInteger();
+
+		private final AtomicInteger disconnect = new AtomicInteger();
+
+
+		public void incrementConnectCount() {
+			this.connect.incrementAndGet();
+		}
+
+		public void incrementConnectedCount() {
+			this.connected.incrementAndGet();
+		}
+
+		public void incrementDisconnectCount() {
+			this.disconnect.incrementAndGet();
+		}
+
+		public String toString() {
+			return connectionHandlers.size() + " sessions, " + relayHost + ":" + relayPort +
+					(isBrokerAvailable() ? " (available)" : " (not available)") +
+					", processed CONNECT(" + this.connect.get() + ")-CONNECTED(" +
+					this.connected.get() + ")-DISCONNECT(" + this.disconnect.get() + ")";
+		}
+	}
+
 }