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/web-handlers.go

@@ -1336,7 +1336,7 @@ func (web *webAPIHandlers) Upload(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	if mustReplicate {
-		scheduleReplication(ctx, objInfo.Clone(), objectAPI, sync)
+		scheduleReplication(ctx, objInfo.Clone(), objectAPI, sync, replication.ObjectReplicationType)
 	}
 
 	reqParams := extractReqParams(r)