Re: Expose lock group leader pid in pg_stat_activity

Hello

I doubt that "Process ID of the lock group leader" is enough for user documentation. I think we need note:
- this field is related to parallel query execution
- leader_pid = pid if process is parallel leader
- leader_pid would point to pid of the leader if process is parallel worker
- leader_pid will be NULL for non-parallel queries or idle sessions

Also patch has no tests. Possible this is normal, not sure how to write a reliable test for this feature.
Patch applies, compiles, pass tests

regards, Sergei