Guard Micrometer stats for NPE: no Micrometer

If there is no Micrometer in classpath, the `timers` and `counters`
are not populated into Graph

* Check for `null` before calling `Supplier`

spring-integration-core/src/main/java/org/springframework/integration/graph/MessageChannelNode.java

@@ -18,12 +18,14 @@ package org.springframework.integration.graph;
 
 import java.util.function.Supplier;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.MessageChannel;
 
 /**
  * Represents a message channel.
  *
  * @author Gary Russell
+ * @author Artem Bilan
  *
  * @since 4.3
  *
@@ -40,8 +42,9 @@ public class MessageChannelNode extends IntegrationNode implements SendTimersAwa
 						: new IntegrationNode.Stats());
 	}
 
+	@Nullable
 	public SendTimers getSendTimers() {
-		return this.sendTimers.get();
+		return this.sendTimers != null ? this.sendTimers.get() : null;
 	}
 
 	@Override

spring-integration-core/src/main/java/org/springframework/integration/graph/MessageHandlerNode.java

@@ -18,12 +18,14 @@ package org.springframework.integration.graph;
 
 import java.util.function.Supplier;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.MessageHandler;
 
 /**
  * Represents a message handler.
  *
  * @author Gary Russell
+ * @author Artem Bilan
  *
  * @since 4.3
  *
@@ -47,8 +49,9 @@ public class MessageHandlerNode extends EndpointNode implements SendTimersAware
 		return this.input;
 	}
 
+	@Nullable
 	public SendTimers getSendTimers() {
-		return this.sendTimers.get();
+		return this.sendTimers != null ? this.sendTimers.get() : null;
 	}
 
 	@Override

spring-integration-core/src/main/java/org/springframework/integration/graph/MessageSourceNode.java

@@ -19,11 +19,13 @@ package org.springframework.integration.graph;
 import java.util.function.Supplier;
 
 import org.springframework.integration.core.MessageSource;
+import org.springframework.lang.Nullable;
 
 /**
  * Represents a message source.
  *
  * @author Gary Russell
+ * @author Artem Bilan
  *
  * @since 4.3
  *
@@ -41,8 +43,9 @@ public class MessageSourceNode extends ErrorCapableEndpointNode implements Recei
 						: new IntegrationNode.Stats());
 	}
 
+	@Nullable
 	public ReceiveCounters getReceiveCounters() {
-		return this.receiveCounters.get();
+		return this.receiveCounters != null ? this.receiveCounters.get() : null;
 	}
 
 	@Override

spring-integration-core/src/main/java/org/springframework/integration/graph/PollableChannelNode.java

@@ -18,12 +18,15 @@ package org.springframework.integration.graph;
 
 import java.util.function.Supplier;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.MessageChannel;
 
 /**
  * Represents a pollable channel.
  *
  * @author Gary Russell
+ * @author Artem Bilan
+ *
  * @since 5.2
  *
  */
@@ -35,8 +38,9 @@ public class PollableChannelNode extends MessageChannelNode implements ReceiveCo
 		super(nodeId, name, channel);
 	}
 
+	@Nullable
 	public ReceiveCounters getReceiveCounters() {
-		return this.receiveCounters.get();
+		return this.receiveCounters != null ? this.receiveCounters.get() : null;
 	}
 
 	@Override