[#1648] writecache: Fix race condition when reporting cache size metrics

There is a race condition when multiple cache operation try to report
the cache size metrics simultaneously. Consider the following example:
- the initial total size of objects stored in the cache size is 2
- worker X deletes an object and reads the cache size, which is 1
- worker Y deletes an object and reads the cache size, which is 0
- worker Y reports the cache size it learnt, which is 0
- worker X reports the cache size it learnt, which is 1

As a result, the observed cache size is 1 (i. e. one object remains
in the cache), which is incorrect because the actual cache size is 0.

To fix this, let's report the metrics periodically in the flush loop.

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/local_object_storage/writecache/flush.go

@@ -87,6 +87,9 @@ func (c *cache) pushToFlushQueue(ctx context.Context, fl *flushLimiter) {
 			}
 
 			c.modeMtx.RUnlock()
+
+			// counter changed by fstree
+			c.estimateCacheSize()
 		case <-ctx.Done():
 			return
 		}