pgsql: Add %P to log_line_prefix for parallel group leader

Add %P to log_line_prefix for parallel group leader

This is useful for monitoring purposes with log parsing. Similarly to
pg_stat_activity, the leader's PID is shown only for active parallel
workers, minimizing the log footprint for the leaders as the equivalent
shared memory field is set as long as a backend is alive.

Author: Justin Pryzby
Reviewed-by: Álvaro Herrera, Michael Paquier, Julien Rouhaud, Tom Lane
Discussion: https://postgr.es/m/20200315111831.GA21492@telsasoft.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b8fdee7d0ca8bd2165d46fb1468f75571b706a01

Modified Files
--------------
doc/src/sgml/config.sgml | 9 ++++++-
src/backend/utils/error/elog.c | 38 +++++++++++++++++++++++++++
src/backend/utils/misc/postgresql.conf.sample | 1 +
3 files changed, 47 insertions(+), 1 deletion(-)