pgsql: Display the leader apply worker's PID for parallel apply workers

Display the leader apply worker's PID for parallel apply workers.

Add leader_pid to pg_stat_subscription. leader_pid is the process ID of
the leader apply worker if this process is a parallel apply worker. If
this field is NULL, it indicates that the process is a leader apply
worker or a synchronization worker. The new column makes it easier to
distinguish parallel apply workers from other kinds of workers and helps
to identify the leader for the parallel workers corresponding to a
particular subscription.

Additionally, update the leader_pid column in pg_stat_activity as well to
display the PID of the leader apply worker for parallel apply workers.

Author: Hou Zhijie
Reviewed-by: Peter Smith, Sawada Masahiko, Amit Kapila, Shveta Mallik
Discussion: https://postgr.es/m/CAA4eK1+wyN6zpaHUkCLorEWNx75MG0xhMwcFhvjqm2KURZEAGw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d540a02a724b9643205abce8c5644a0f0908f6e3

Modified Files
--------------
doc/src/sgml/logical-replication.sgml              |  3 +-
doc/src/sgml/monitoring.sgml                       | 36 ++++++++----
src/backend/catalog/system_views.sql               |  1 +
.../replication/logical/applyparallelworker.c      |  6 +-
src/backend/replication/logical/launcher.c         | 64 ++++++++++++++++------
src/backend/utils/adt/pgstatfuncs.c                | 21 +++++--
src/include/catalog/catversion.h                   |  2 +-
src/include/catalog/pg_proc.dat                    |  6 +-
src/include/replication/logicallauncher.h          |  2 +
src/include/replication/worker_internal.h          |  4 +-
src/test/regress/expected/rules.out                |  3 +-
11 files changed, 105 insertions(+), 43 deletions(-)