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/erasure-server-pool.go

@@ -433,7 +433,7 @@ func (z *erasureServerPools) StorageInfo(ctx context.Context) (StorageInfo, []er
 	return storageInfo, errs
 }
 
-func (z *erasureServerPools) NSScanner(ctx context.Context, bf *bloomFilter, updates chan<- DataUsageInfo) error {
+func (z *erasureServerPools) NSScanner(ctx context.Context, bf *bloomFilter, updates chan<- madmin.DataUsageInfo) error {
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
@@ -448,7 +448,7 @@ func (z *erasureServerPools) NSScanner(ctx context.Context, bf *bloomFilter, upd
 	}
 
 	if len(allBuckets) == 0 {
-		updates <- DataUsageInfo{} // no buckets found update data usage to reflect latest state
+		updates <- madmin.DataUsageInfo{} // no buckets found update data usage to reflect latest state
 		return nil
 	}