[CELEBORN-1491] introduce flusher working queue size metric

### What changes were proposed in this pull request?
Add metrics about flusher working queue size.

### Why are the changes needed?
To show if there is an accumulation of flush tasks.

### Does this PR introduce _any_ user-facing change?
NO.

### How was this patch tested?
GA.

Closes #2598 from FMX/b1491.

Authored-by: mingji <fengmingxiao.fmx@alibaba-inc.com>
Signed-off-by: Shuang <lvshuang.xjs@alibaba-inc.com>