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/gateway-unsupported.go

@@ -47,7 +47,7 @@ func (a GatewayUnsupported) LocalStorageInfo(ctx context.Context) (StorageInfo,
 }
 
 // NSScanner - scanner is not implemented for gateway
-func (a GatewayUnsupported) NSScanner(ctx context.Context, bf *bloomFilter, updates chan<- DataUsageInfo) error {
+func (a GatewayUnsupported) NSScanner(ctx context.Context, bf *bloomFilter, updates chan<- madmin.DataUsageInfo) error {
 	logger.CriticalIf(ctx, errors.New("not implemented"))
 	return NotImplemented{}
 }