[#312] metrics: Add writecache metrcis

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

pkg/metrics/node.go

@@ -10,6 +10,8 @@ type NodeMetrics struct {
 	stateMetrics
 	replicatorMetrics
 	epoch metric[prometheus.Gauge]
+
+	writeCacheMetrics *writeCacheMetrics
 }
 
 func NewNodeMetrics() *NodeMetrics {
@@ -33,12 +35,16 @@ func NewNodeMetrics() *NodeMetrics {
 	})
 	mustRegister(epoch)
 
+	writeCacheMetrics := newWriteCacheMetrics()
+	writeCacheMetrics.register()
+
 	return &NodeMetrics{
 		objectServiceMetrics: objectService,
 		engineMetrics:        engine,
 		stateMetrics:         state,
 		replicatorMetrics:    replicator,
 		epoch:                epoch,
+		writeCacheMetrics:    writeCacheMetrics,
 	}
 }
 
@@ -46,3 +52,11 @@ func NewNodeMetrics() *NodeMetrics {
 func (m *NodeMetrics) SetEpoch(epoch uint64) {
 	m.epoch.value.Set(float64(epoch))
 }
+
+// WriteCache returns WriteCache metrics.
+func (m *NodeMetrics) WriteCache() WriteCacheMetrics {
+	if m == nil {
+		return nil
+	}
+	return m.writeCacheMetrics
+}