fix for issue 2976 (#3313)

fix for null pointer exception in ZookeeperLogger

discovery/zookeeper/zookeeper.go

@@ -68,7 +68,7 @@ func NewDiscovery(
 	}
 
 	conn, _, err := zk.Connect(srvs, timeout)
-	conn.SetLogger(treecache.ZookeeperLogger{})
+	conn.SetLogger(treecache.NewZookeeperLogger(logger))
 	if err != nil {
 		return nil
 	}

util/treecache/treecache.go

@@ -49,6 +49,11 @@ type ZookeeperLogger struct {
 	logger log.Logger
 }
 
+// NewZookeeperLogger is a constructor for ZookeeperLogger
+func NewZookeeperLogger(logger log.Logger) ZookeeperLogger {
+	return ZookeeperLogger{logger: logger}
+}
+
 // Implements zk.Logger
 func (zl ZookeeperLogger) Printf(s string, i ...interface{}) {
 	level.Info(zl.logger).Log("msg", fmt.Sprintf(s, i...))