Add data usage collect with its new admin API (#8553)

Admin data usage info API returns the following

(Only FS & XL, for now)

- Number of buckets
- Number of objects
- The total size of objects
- Objects histogram
- Bucket sizes

cmd/object-api-common.go

@@ -450,3 +450,26 @@ func listObjects(ctx context.Context, obj ObjectLayer, bucket, prefix, marker, d
 	// Success.
 	return result, nil
 }
+
+// Fetch the histogram interval corresponding
+// to the passed object size.
+func objSizeToHistoInterval(usize uint64) string {
+	size := int64(usize)
+
+	var interval objectHistogramInterval
+	for _, interval = range ObjectsHistogramIntervals {
+		var cond1, cond2 bool
+		if size >= interval.start || interval.start == -1 {
+			cond1 = true
+		}
+		if size <= interval.end || interval.end == -1 {
+			cond2 = true
+		}
+		if cond1 && cond2 {
+			return interval.name
+		}
+	}
+
+	// This would be the last element of histogram intervals
+	return interval.name
+}