Include MillisBehindLatest in Record for ScanFunc (#124)

consumer.go

@@ -20,7 +20,8 @@ import (
 // extends to include the shard id.
 type Record struct {
 	*kinesis.Record
-	ShardID string
+	ShardID            string
+	MillisBehindLatest *int64
 }
 
 // New creates a kinesis consumer with default settings. Use Option to override
@@ -184,7 +185,7 @@ func (c *Consumer) ScanShard(ctx context.Context, shardID string, fn ScanFunc) e
 				case <-ctx.Done():
 					return nil
 				default:
-					err := fn(&Record{r, shardID})
+					err := fn(&Record{r, shardID, resp.MillisBehindLatest})
 					if err != nil && err != ErrSkipCheckpoint {
 						return err
 					}