fix(inputs.kafka_consumer): use mutex for parser

Using parsers like json_v2 will result in undesired parser results. This
adds a mutex to calls to the parser to prevent side effects from calling
the parser in parallel.

plugins/inputs/kafka_consumer/kafka_consumer.go

@@ -456,7 +456,9 @@ func (h *ConsumerGroupHandler) Handle(session sarama.ConsumerGroupSession, msg *
 			len(msg.Value), h.MaxMessageLen)
 	}
 
+	h.mu.Lock()
 	metrics, err := h.parser.Parse(msg.Value)
+	h.mu.Unlock()
 	if err != nil {
 		h.release()
 		return err