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/user-commands.go

@@ -35,9 +35,9 @@ type AccountAccess struct {
 	Write bool `json:"write"`
 }
 
-// BucketUsageInfo represents bucket usage of a bucket, and its relevant
+// BucketAccessInfo represents bucket usage of a bucket, and its relevant
 // access type for an account
-type BucketUsageInfo struct {
+type BucketAccessInfo struct {
 	Name    string        `json:"name"`
 	Size    uint64        `json:"size"`
 	Created time.Time     `json:"created"`
@@ -49,7 +49,7 @@ type BucketUsageInfo struct {
 type AccountInfo struct {
 	AccountName string
 	Policy      iampolicy.Policy
-	Buckets     []BucketUsageInfo
+	Buckets     []BucketAccessInfo
 }
 
 // AccountInfo returns the usage info for the authenticating account.