fetch bucket replication stats across peers in single call (#14956)

current implementation relied on recursively calling one bucket
at a time across all peers, this would be very slow and chatty
when there are 100's of buckets which would mean 100*peerCount
amount of network operations.

This PR attempts to reduce this entire call into `peerCount`
amount of network calls only. This functionality addresses also a
concern where the Prometheus metrics would significantly slow
down when one of the peers is offline.

cmd/bucket-stats.go

@@ -51,6 +51,9 @@ func (rl *ReplicationLatency) update(size int64, duration time.Duration) {
 	rl.UploadHistogram.Add(size, duration)
 }
 
+// BucketStatsMap captures bucket statistics for all buckets
+type BucketStatsMap map[string]BucketStats
+
 // BucketStats bucket statistics
 type BucketStats struct {
 	ReplicationStats BucketReplicationStats