Use 'localhost' as default 'server' tag

Resolves #4381

plugins/inputs/zookeeper/dev/docker-compose.yml

@@ -0,0 +1,17 @@
+version: '3'
+services:
+  zoo:
+    image: zookeeper
+
+  telegraf:
+    image: glinton/scratch
+    volumes:
+      - ./telegraf.conf:/telegraf.conf
+      - ../../../../telegraf:/telegraf
+    depends_on:
+      - zoo
+    entrypoint:
+      - /telegraf
+      - --config
+      - /telegraf.conf
+    network_mode: service:zoo

plugins/inputs/zookeeper/dev/telegraf.conf

@@ -0,0 +1,9 @@
+[agent]
+  interval="1s"
+  flush_interval="1s"
+
+[[inputs.zookeeper]]
+  servers = [":2181"]
+
+[[outputs.file]]
+  files = ["stdout"]

plugins/inputs/zookeeper/zookeeper.go

@@ -158,8 +158,14 @@ func (z *Zookeeper) gatherServer(ctx context.Context, address string, acc telegr
 			}
 		}
 	}
+
+	srv := "localhost"
+	if service[0] != "" {
+		srv = service[0]
+	}
+
 	tags := map[string]string{
-		"server": service[0],
+		"server": srv,
 		"port":   service[1],
 		"state":  zookeeper_state,
 	}