pgsql: Add information of total data processed to replication slot stat

Add information of total data processed to replication slot stats.

This adds the statistics about total transactions count and total
transaction data logically sent to the decoding output plugin from
ReorderBuffer. Users can query the pg_stat_replication_slots view to check
these stats.

Suggested-by: Andres Freund
Author: Vignesh C and Amit Kapila
Reviewed-by: Sawada Masahiko, Amit Kapila
Discussion: https://postgr.es/m/20210319185247.ldebgpdaxsowiflw@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f5fc2f5b23d1b1dff60f8ca5dc211161df47eda4

Modified Files
--------------
contrib/test_decoding/Makefile | 2 +
contrib/test_decoding/expected/stats.out | 79 ++++++++++++++++++-------
contrib/test_decoding/sql/stats.sql | 48 ++++++++++-----
contrib/test_decoding/t/001_repl_stats.pl | 76 ++++++++++++++++++++++++
doc/src/sgml/monitoring.sgml | 25 ++++++++
src/backend/catalog/system_views.sql | 2 +
src/backend/postmaster/pgstat.c | 6 ++
src/backend/replication/logical/logical.c | 18 +++---
src/backend/replication/logical/reorderbuffer.c | 21 +++++++
src/backend/utils/adt/pgstatfuncs.c | 8 ++-
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 6 +-
src/include/pgstat.h | 4 ++
src/include/replication/reorderbuffer.h | 7 +++
src/test/regress/expected/rules.out | 4 +-
15 files changed, 256 insertions(+), 52 deletions(-)