[consensus] fix units for log_reader_read_batch_latency metric

This is a follow-up to 9689272e45bdeabe89fdbf675d2e49e71837feb8.

Change-Id: Ie769021a3ab21a5447b682443ebc756d7ff0cb1a
Reviewed-on: http://gerrit.cloudera.org:8080/18961
Reviewed-by: Yifan Zhang <chinazhangyifan@163.com>
Tested-by: Kudu Jenkins
1 file changed