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/storage-datatypes.go

@@ -47,7 +47,7 @@ type DiskInfo struct {
 // the number of calls of each API and the moving average of
 // the duration of each API.
 type DiskMetrics struct {
-	APILatencies map[string]string `json:"apiLatencies,omitempty"`
+	APILatencies map[string]uint64 `json:"apiLatencies,omitempty"`
 	APICalls     map[string]uint64 `json:"apiCalls,omitempty"`
 }