[#1439] object: Sort nodes by priority metrics to compute GET request

Signed-off-by: Anton Nikiforov <an.nikiforov@yadro.com>

pkg/services/object_manager/placement/metrics.go

@@ -0,0 +1,43 @@
+package placement
+
+import (
+	"errors"
+	"strings"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/netmap"
+)
+
+const (
+	attrPrefix = "$attribute:"
+)
+
+type Metric interface {
+	CalculateValue(*netmap.NodeInfo, *netmap.NodeInfo) int
+}
+
+func ParseMetric(raw string) (Metric, error) {
+	if attr, found := strings.CutPrefix(raw, attrPrefix); found {
+		return NewAttributeMetric(attr), nil
+	}
+	return nil, errors.New("unsupported priority metric")
+}
+
+// attributeMetric describes priority metric based on attribute.
+type attributeMetric struct {
+	attribute string
+}
+
+// CalculateValue return [0] if from and to contains attribute attributeMetric.attribute and
+// the value of attribute is the same. In other case return [1].
+func (am *attributeMetric) CalculateValue(from *netmap.NodeInfo, to *netmap.NodeInfo) int {
+	fromAttr := from.Attribute(am.attribute)
+	toAttr := to.Attribute(am.attribute)
+	if len(fromAttr) > 0 && len(toAttr) > 0 && fromAttr == toAttr {
+		return 0
+	}
+	return 1
+}
+
+func NewAttributeMetric(attr string) Metric {
+	return &attributeMetric{attribute: attr}
+}