[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
diff --git a/src/kudu/consensus/log_reader.cc b/src/kudu/consensus/log_reader.cc
index 24a4739..40743e0 100644
--- a/src/kudu/consensus/log_reader.cc
+++ b/src/kudu/consensus/log_reader.cc
@@ -22,6 +22,7 @@
 #include <memory>
 #include <mutex>
 #include <ostream>
+#include <type_traits>
 
 #include <glog/logging.h>
 
@@ -51,7 +52,7 @@
                       kudu::MetricLevel::kDebug);
 
 METRIC_DEFINE_histogram(tablet, log_reader_read_batch_latency, "Log Read Latency",
-                        kudu::MetricUnit::kBytes,
+                        kudu::MetricUnit::kMicroseconds,
                         "Microseconds spent reading log entry batches",
                         kudu::MetricLevel::kInfo,
                         60000000LU, 2);