pgsql: Add leader_pid to pg_stat_activity

Add leader_pid to pg_stat_activity

This new field tracks the PID of the group leader used with parallel
query. For parallel workers and the leader, the value is set to the
PID of the group leader. So, for the group leader, the value is the
same as its own PID. Note that this reflects what PGPROC stores in
shared memory, so as leader_pid is NULL if a backend has never been
involved in parallel query. If the backend is using parallel query or
has used it at least once, the value is set until the backend exits.

Author: Julien Rouhaud
Reviewed-by: Sergei Kornilov, Guillaume Lelarge, Michael Paquier, Tomas
Vondra
Discussion: https://postgr.es/m/CAOBaU_Yy5bt0vTPZ2_LUM6cUcGeqmYNoJ8-Rgto+c2+w3defYA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b025f32e0b5d7668daec9bfa957edf3599f4baa8

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 12 ++++++++++
src/backend/catalog/system_views.sql | 1 +
src/backend/utils/adt/pgstatfuncs.c | 44 +++++++++++++++++++++---------------
src/include/catalog/pg_proc.dat | 6 ++---
src/test/regress/expected/rules.out | 9 ++++----
5 files changed, 47 insertions(+), 25 deletions(-)