GH-927 Fixed GMT formatting issue

Fixed DateFormat definition to include 'ISO 8601 time zone' designator (see https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html)
Added constamt TimeZone to dateFormat
Added test

GH-927 fixed imports

spring-cloud-stream-metrics/src/main/java/org/springframework/cloud/stream/metrics/config/MetricJsonSerializer.java

@@ -21,6 +21,7 @@ import java.text.DateFormat;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
 import java.util.Date;
+import java.util.TimeZone;
 
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.core.JsonParser;
@@ -36,11 +37,16 @@ import org.springframework.boot.jackson.JsonComponent;
 
 /**
  * @author Vinicius Carvalho
+ * @author Oleg Zhurakousky
  */
 @JsonComponent
 public class MetricJsonSerializer {
 
-	final static DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
+	private final static DateFormat df;
+	static {
+		df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSX");
+		df.setTimeZone(TimeZone.getTimeZone("GMT"));
+	}
 
 	public static class Serializer extends JsonSerializer<Metric<?>> {
 

spring-cloud-stream-metrics/src/test/java/org/springframework/cloud/stream/metrics/config/MetricJsonSerializerTests.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.stream.metrics.config;
+
+import java.io.StringWriter;
+import java.util.Date;
+
+import com.fasterxml.jackson.core.JsonFactory;
+import com.fasterxml.jackson.core.JsonGenerator;
+
+import org.json.JSONObject;
+import org.junit.Test;
+
+import org.springframework.boot.actuate.metrics.Metric;
+import org.springframework.cloud.stream.metrics.config.MetricJsonSerializer.Serializer;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * @author Oleg Zhurakousky
+ */
+public class MetricJsonSerializerTests {
+
+	@Test
+	public void validateAlwaysGMTDateAndFormat() throws Exception {	
+		Date date = new Date(1493060197188L); // Mon Apr 24 14:56:37 EDT 2017
+		Metric<Number> metric = new Metric<Number>("Hello", 123, date);	
+		
+		JsonFactory factory = new JsonFactory();
+		StringWriter writer = new StringWriter();	
+		JsonGenerator jsonGenerator = factory.createGenerator(writer);
+		Serializer ser = new Serializer();	
+		ser.serialize(metric, jsonGenerator, null);
+		jsonGenerator.flush();
+		
+		JSONObject json = new JSONObject(writer.toString());
+		String serializedTimestamp = json.getString("timestamp");
+		assertEquals("2017-04-24T18:56:37.188Z", serializedTimestamp);
+	}
+}