Support array (multi valued) gauges.

src/main/java/org/elasticsearch/metrics/MetricsElasticsearchModule.java

@@ -32,6 +32,7 @@
 
 import java.io.IOException;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Locale;
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
@@ -53,7 +54,7 @@ private static void writeAdditionalFields(final Map<String, Object> additionalFi
             }
         }
     }
-
+    
     private static class GaugeSerializer extends StdSerializer<JsonGauge> {
         private final String timestampFieldname;
         private final Map<String, Object> additionalFields;
@@ -74,7 +75,16 @@ public void serialize(JsonGauge gauge,
             final Object value;
             try {
                 value = gauge.value().getValue();
-                json.writeObjectField("value", value);
+                if (value instanceof Iterable) { // Check if writing a single value or an array
+                	Iterable<?> iterable = (Iterable<?>) value;
+                	json.writeArrayFieldStart("value");
+                	for (Object valueItem : iterable) {
+                		json.writeObject(valueItem);
+                	}
+                	json.writeEndArray();
+                } else {
+                	json.writeObjectField("value", value);	
+                }
             } catch (RuntimeException e) {
                 json.writeObjectField("error", e.toString());
             }

src/test/java/org/elasticsearch/metrics/MetricsElasticsearchModuleTest.java

@@ -0,0 +1,66 @@
+package org.elasticsearch.metrics;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.concurrent.TimeUnit;
+
+import org.elasticsearch.metrics.JsonMetrics.JsonGauge;
+import org.elasticsearch.metrics.JsonMetrics.JsonMetric;
+import org.junit.Test;
+
+import com.codahale.metrics.Gauge;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import static org.junit.Assert.*;
+
+/**
+ * Tests if value is an array.
+ * @author static-max
+ *
+ */
+public class ElasticsearchReporterTestTest {
+
+	@Test
+	public void test() throws JsonProcessingException {
+		ObjectMapper objectMapper = new ObjectMapper();
+		objectMapper.registerModule(new MetricsElasticsearchModule(TimeUnit.MINUTES, TimeUnit.MINUTES, "@timestamp", null));
+
+		Gauge<String> gaugeString = new Gauge<String>() {
+			@Override
+			public String getValue() {
+				return "STRING VALUE";
+			}
+		};
+
+		/**
+		 * Used for deadlocks in metrics-jvm ThreadStatesGaugeSet.class.
+		 */
+		Gauge<Set<String>> gaugeStringSet = new Gauge<Set<String>>() {
+			@Override
+			public Set<String> getValue() {
+				HashSet<String> testSet = new HashSet<>();
+				testSet.add("1");
+				testSet.add("2");
+				testSet.add("3");
+				return testSet;
+			}
+		};
+
+
+		JsonMetric<?> jsonMetricString = new JsonGauge("string", Long.MAX_VALUE, gaugeString);
+		JsonMetric<?> jsonMetricStringSet = new JsonGauge("string", Long.MAX_VALUE, gaugeStringSet);
+
+		JsonNode stringNode = objectMapper.valueToTree(jsonMetricString);
+		assertTrue(stringNode.get("value").isTextual());
+		assertFalse(stringNode.get("value").isNumber());
+
+		JsonNode stringSetNode = objectMapper.valueToTree(jsonMetricStringSet);
+		assertTrue(stringSetNode.get("value").isArray());
+		assertTrue(stringSetNode.get("value").get(0).isTextual());
+		assertTrue(stringSetNode.get("value").get(1).isTextual());
+		assertTrue(stringSetNode.get("value").get(2).isTextual());
+	}
+
+}