publish storage API latency through node metrics (#14117)

Publish storage functions latency to help compare the performance 
of different disks in a single deployment.

e.g.:
```
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/1",server="localhost:9001"} 226
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/2",server="localhost:9002"} 1180
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/3",server="localhost:9003"} 1183
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/4",server="localhost:9004"} 1625
```

cmd/bucket-stats.go

@@ -38,7 +38,7 @@ func (rl ReplicationLatency) merge(other ReplicationLatency) (newReplLatency Rep
 // Get upload latency of each object size range
 func (rl ReplicationLatency) getUploadLatency() (ret map[string]uint64) {
 	ret = make(map[string]uint64)
-	avg := rl.UploadHistogram.GetAvg()
+	avg := rl.UploadHistogram.GetAvgData()
 	for k, v := range avg {
 		// Convert nanoseconds to milliseconds
 		ret[sizeTagToString(k)] = v.avg() / uint64(time.Millisecond)