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>
