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>

cmd/bucket-metadata-sys.go

@@ -169,7 +169,10 @@ func (sys *BucketMetadataSys) Update(bucket string, configFile string, configDat
 		}
 		meta.ReplicationConfigXML = configData
 	case bucketTargetsFile:
-		meta.BucketTargetsConfigJSON, meta.BucketTargetsConfigMetaJSON, err = encryptBucketMetadata(meta.Name, configData, crypto.Context{bucket: meta.Name, bucketTargetsFile: bucketTargetsFile})
+		meta.BucketTargetsConfigJSON, meta.BucketTargetsConfigMetaJSON, err = encryptBucketMetadata(meta.Name, configData, crypto.Context{
+			bucket:            meta.Name,
+			bucketTargetsFile: bucketTargetsFile,
+		})
 		if err != nil {
 			return fmt.Errorf("Error encrypting bucket target metadata %w", err)
 		}