feat: Add Metrics V3 API (#19068)

Metrics v3 is mainly a reorganization of metrics into smaller groups of
metrics and the removal of internal aggregation of metrics received from
peer nodes in a MinIO cluster.

This change adds the endpoint `/minio/metrics/v3` as the top-level metrics
endpoint and under this, various sub-endpoints are implemented. These
are currently documented in `docs/metrics/v3.md`

The handler will serve metrics at any path
`/minio/metrics/v3/PATH`, as follows:

when PATH is a sub-endpoint listed above => serves the group of
metrics under that path; or when PATH is a (non-empty) parent 
directory of the sub-endpoints listed above => serves metrics
from each child sub-endpoint of PATH. otherwise, returns a no 
resource found error

All available metrics are listed in the `docs/metrics/v3.md`. More will
be added subsequently.

cmd/http-stats.go

@@ -269,6 +269,28 @@ func (s *bucketConnStats) getS3InOutBytes() map[string]inOutBytes {
 	return bucketStats
 }
 
+// Return S3 total input/output bytes for each
+func (s *bucketConnStats) getBucketS3InOutBytes(buckets []string) map[string]inOutBytes {
+	s.RLock()
+	defer s.RUnlock()
+
+	if len(s.stats) == 0 || len(buckets) == 0 {
+		return nil
+	}
+
+	bucketStats := make(map[string]inOutBytes, len(buckets))
+	for _, bucket := range buckets {
+		if stats, ok := s.stats[bucket]; ok {
+			bucketStats[bucket] = inOutBytes{
+				In:  stats.s3InputBytes,
+				Out: stats.s3OutputBytes,
+			}
+		}
+	}
+
+	return bucketStats
+}
+
 // delete metrics once bucket is deleted.
 func (s *bucketConnStats) delete(bucket string) {
 	s.Lock()