pgsql: pg_stat_statements: Add wal_buffers_full

pg_stat_statements: Add wal_buffers_full

wal_buffers_full tracks the number of times WAL buffers become full,
giving hints to be able to tune the GUC wal_buffers.

Up to now, this information was only available in pg_stat_wal. With
this field available in WalUsage since eaf502747bac, exposing it in
pg_stat_statements is straight-forward, and it offers more granularity
at query level.

pg_stat_statements does not need a version bump as one has been done in
commit cf54a2c00254 for this development cycle.

Author: Bertrand Drouvot
Reviewed-by: Ilia Evdokimov
Discussion: https://postgr.es/m/Z6SOha5YFFgvpwQY@ip-10-97-1-34.eu-west-3.compute.internal

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ce5bcc4a9f26484746f82d23584c8e342cba9b10

Modified Files
--------------
contrib/pg_stat_statements/expected/oldextversions.out | 1 +
contrib/pg_stat_statements/pg_stat_statements--1.11--1.12.sql | 1 +
contrib/pg_stat_statements/pg_stat_statements.c | 10 ++++++++--
doc/src/sgml/pgstatstatements.sgml | 9 +++++++++
4 files changed, 19 insertions(+), 2 deletions(-)