Various improvements in replication (#11949)

- collect real time replication metrics for prometheus.
- add pending_count, failed_count metric for total pending/failed replication operations.

- add API to get replication metrics

- add MRF worker to handle spill-over replication operations

- multiple issues found with replication
- fixes an issue when client sends a bucket
 name with `/` at the end from SetRemoteTarget
 API call make sure to trim the bucket name to 
 avoid any extra `/`.

- hold write locks in GetObjectNInfo during replication
  to ensure that object version stack is not overwritten
  while reading the content.

- add additional protection during WriteMetadata() to
  ensure that we always write a valid FileInfo{} and avoid
  ever writing empty FileInfo{} to the lowest layers.

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

pkg/madmin/update-commands.go

@@ -20,7 +20,6 @@ package madmin
 import (
 	"context"
 	"encoding/json"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 )
@@ -53,10 +52,9 @@ func (adm *AdminClient) ServerUpdate(ctx context.Context, updateURL string) (us
 		return us, httpRespToErrorResponse(resp)
 	}
 
-	buf, err := ioutil.ReadAll(resp.Body)
-	if err != nil {
+	if err = json.NewDecoder(resp.Body).Decode(&us); err != nil {
 		return us, err
 	}
-	err = json.Unmarshal(buf, &us)
-	return us, err
+
+	return us, nil
 }