replication lag can only be displayed to the second

[csuzhangxc]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

   set enable-heartbeat: true for task.yaml.

2. What did you expect to see?

   the replication lag can be a valid less than 1s.

3. What did you see instead?

   thre replication lag always >= 1s.

4. Versions of the cluster

   • DM version (run dmctl -V or dm-worker -V or dm-master -V):

     v1.0.5
     

[csuzhangxc]

we use UTC_TIMESTAMP() to generate the current timestamp before, but it will get a fractional seconds precision with 0.