add additional metrics per disk API latency, API call counts #11250)

```
mc admin info --json
```

provides these details, for now, we shall eventually 
expose this at Prometheus level eventually. 

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/metacache-walk.go

@@ -25,7 +25,6 @@ import (
 	"sort"
 	"strconv"
 	"strings"
-	"sync/atomic"
 
 	"github.com/gorilla/mux"
 	"github.com/minio/minio/cmd/logger"
@@ -57,11 +56,6 @@ type WalkDirOptions struct {
 // WalkDir will traverse a directory and return all entries found.
 // On success a sorted meta cache stream will be returned.
 func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writer) error {
-	atomic.AddInt32(&s.activeIOCount, 1)
-	defer func() {
-		atomic.AddInt32(&s.activeIOCount, -1)
-	}()
-
 	// Verify if volume is valid and it exists.
 	volumeDir, err := s.getVolDir(opts.Bucket)
 	if err != nil {
@@ -266,6 +260,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 }
 
 func (p *xlStorageDiskIDCheck) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writer) error {
+	defer p.updateStorageMetrics(storageMetricWalkDir)()
 	if err := p.checkDiskStale(); err != nil {
 		return err
 	}