os/bluestore: add some slow count for bluestore

os/bluestore: add some slow count for bluestore

Add slow count as below:
- l_bluestore_slow_aio_wait_count
- l_bluestore_slow_committed_kv_count
- l_bluestore_slow_read_onode_meta_count
- l_bluestore_slow_read_wait_aio_count

We can get a count while bluestore happens slowly,
in some cases, this is more useful than average latency.
Add it to prometheus, we can get it from the dashboard.

Signed-off-by: Yite Gu <yitegu0@gmail.com>