updated the documentation

sarama.go

@@ -58,6 +58,14 @@ Producer related metrics:
 	| compression-ratio-for-topic-<topic>       | histogram  | Distribution of the compression ratio times 100 of record batches for a given topic  |
 	+-------------------------------------------+------------+--------------------------------------------------------------------------------------+
 
+Consumer related metrics:
+
+	+-------------------------------------------+------------+--------------------------------------------------------------------------------------+
+	| Name                                      | Type       | Description                                                                          |
+	+-------------------------------------------+------------+--------------------------------------------------------------------------------------+
+	| consumer-batch-size						| histogram  | Distribution of the number of messages in a batch                                    |
+	+-------------------------------------------+------------+--------------------------------------------------------------------------------------+
+
 */
 package sarama