metrics: fix tso alert (#4282)
metrics: fix tso alert (#4282)

* fix

Signed-off-by: lhy1024 <admin@liudos.us>

* close #4283

Signed-off-by: lhy1024 <admin@liudos.us>

Co-authored-by: Ti Chi Robot <040e6455cde1c91e876ec337e785ce913eacba35@tidb.io>
pingcap/pd,pingcap/pd