Fix Memory Leak in Kafka Producer

This commit addresses an issue where the sarama kafka library
leaks memory when a connection closes unless the metrics
configuration that was passed to new connection is also closed.

Fixes #613

internal/endpoint/kafka.go

@@ -24,6 +24,7 @@ type KafkaConn struct {
 	mu   sync.Mutex
 	ep   Endpoint
 	conn sarama.SyncProducer
+	cfg  *sarama.Config
 	ex   bool
 	t    time.Time
 }
@@ -47,6 +48,8 @@ func (conn *KafkaConn) close() {
 	if conn.conn != nil {
 		conn.conn.Close()
 		conn.conn = nil
+		conn.cfg.MetricRegistry.UnregisterAll()
+		conn.cfg = nil
 	}
 }
 
@@ -72,6 +75,7 @@ func (conn *KafkaConn) Send(msg string) error {
 			log.Debugf("building kafka tls config")
 			tlsConfig, err := newKafkaTLSConfig(conn.ep.Kafka.CertFile, conn.ep.Kafka.KeyFile, conn.ep.Kafka.CACertFile)
 			if err != nil {
+				cfg.MetricRegistry.UnregisterAll()
 				return err
 			}
 			cfg.Net.TLS.Enable = true
@@ -103,10 +107,12 @@ func (conn *KafkaConn) Send(msg string) error {
 
 		c, err := sarama.NewSyncProducer([]string{uri}, cfg)
 		if err != nil {
+			cfg.MetricRegistry.UnregisterAll()
 			return err
 		}
 
 		conn.conn = c
+		conn.cfg = cfg
 	}
 
 	// parse json again to get out info for our kafka key