adding the tags in the graylog output plugin

closes #1861

CHANGELOG.md

@@ -11,6 +11,7 @@ continue sending logs to /var/log/telegraf/telegraf.log.
 
 ### Features
 
+- [#1861](https://github.com/influxdata/telegraf/pull/1861): adding the tags in the graylog output plugin
 - [#1732](https://github.com/influxdata/telegraf/pull/1732): Telegraf systemd service, log to journal.
 - [#1782](https://github.com/influxdata/telegraf/pull/1782): Allow numeric and non-string values for tag_keys.
 - [#1694](https://github.com/influxdata/telegraf/pull/1694): Adding Gauge and Counter metric types.

plugins/outputs/graylog/graylog.go

@@ -226,9 +226,14 @@ func serialize(metric telegraf.Metric) ([]string, error) {
 		m["host"] = host
 	}
 
+	for key, value := range metric.Tags() {
+		if key != "host" {
+			m["_"+key] = value
+		}
+	}
+
 	for key, value := range metric.Fields() {
-		nkey := fmt.Sprintf("_%s", key)
-		m[nkey] = value
+		m["_"+key] = value
 	}
 
 	serialized, err := ejson.Marshal(m)