Add WAL data to backend statistics

This commit adds per-backend WAL statistics, providing the same
information as pg_stat_wal, except that it is now possible to know how
much WAL activity is happening in each backend rather than an overall
aggregate of all the activity.  Like pg_stat_wal, the implementation
relies on pgWalUsage, tracking the difference of activity between two
reports to pgstats.

This data can be retrieved with a new system function called
pg_stat_get_backend_wal(), that returns one tuple based on the PID
provided in input.  Like pg_stat_get_backend_io(), this is useful when
joined with pg_stat_activity to get a live picture of the WAL generated
for each running backend, showing how the activity is [un]balanced.

pgstat_flush_backend() gains a new flag value, able to control the flush
of the WAL stats.

This commit relies mostly on the infrastructure provided by
9aea73fc61, that has introduced backend statistics.

Bump catalog version.  A bump of PGSTAT_FILE_FORMAT_ID is not required,
as backend stats do not persist on disk.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal

src/test/regress/expected/stats.out

@@ -908,8 +908,11 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 
 -- Test pg_stat_checkpointer checkpointer-related stats, together with pg_stat_wal
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
--- Test pg_stat_wal (and make a temp table so our temp schema exists)
+-- Test pg_stat_wal
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal()
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+-- Make a temp table so our temp schema exists
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -929,6 +932,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.

src/test/regress/sql/stats.sql

@@ -426,9 +426,13 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 -- Test pg_stat_checkpointer checkpointer-related stats, together with pg_stat_wal
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 
--- Test pg_stat_wal (and make a temp table so our temp schema exists)
+-- Test pg_stat_wal
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal()
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
+-- Make a temp table so our temp schema exists
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -441,6 +445,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.