[#312] wc: Add metrics

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

pkg/local_object_storage/writecache/writecache.go

@@ -103,6 +103,7 @@ func New(opts ...Option) Cache {
 			maxBatchSize:    bbolt.DefaultMaxBatchSize,
 			maxBatchDelay:   bbolt.DefaultMaxBatchDelay,
 			openFile:        os.OpenFile,
+			metrics:         &metricsStub{},
 		},
 	}
 
@@ -140,6 +141,7 @@ func (c *cache) Open(readOnly bool) error {
 
 // Init runs necessary services.
 func (c *cache) Init() error {
+	c.metrics.SetMode(c.mode)
 	c.runFlushLoop()
 	return nil
 }