[#312] wc: Add metrics

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/writecache/metrics.go

@@ -0,0 +1,42 @@
+package writecache
+
+import (
+	"time"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard/mode"
+)
+
+type storageType string
+
+const (
+	storageTypeUndefined storageType = "null"
+	storageTypeDB        storageType = "db"
+	storageTypeFSTree    storageType = "fstree"
+)
+
+type Metrics interface {
+	Get(d time.Duration, success bool, st storageType)
+	Delete(d time.Duration, success bool, st storageType)
+	Put(d time.Duration, success bool, st storageType)
+	Flush(success bool, st storageType)
+	Evict(st storageType)
+
+	Estimate(db, fstree uint64)
+	SetMode(m mode.Mode)
+}
+
+type metricsStub struct{}
+
+func (s *metricsStub) Get(time.Duration, bool, storageType) {}
+
+func (s *metricsStub) Delete(time.Duration, bool, storageType) {}
+
+func (s *metricsStub) Put(time.Duration, bool, storageType) {}
+
+func (s *metricsStub) Estimate(uint64, uint64) {}
+
+func (s *metricsStub) SetMode(mode.Mode) {}
+
+func (s *metricsStub) Flush(bool, storageType) {}
+
+func (s *metricsStub) Evict(storageType) {}