pgsql: Re-add GUC track_wal_io_timing

Re-add GUC track_wal_io_timing

This commit is a rework of 2421e9a51d20, about which Andres Freund has
raised some concerns as it is valuable to have both track_io_timing and
track_wal_io_timing in some cases, as the WAL write and fsync paths can
be a major bottleneck for some workloads. Hence, it can be relevant to
not calculate the WAL timings in environments where pg_test_timing
performs poorly while capturing some IO data under track_io_timing for
the non-WAL IO paths. The opposite can be also true: it should be
possible to disable the non-WAL timings and enable the WAL timings (the
previous GUC setups allowed this possibility).

track_wal_io_timing is added back in this commit, controlling if WAL
timings should be calculated in pg_stat_io for the read, fsync and write
paths, as done previously with pg_stat_wal. pg_stat_wal previously
tracked only the sync and write parts (now removed), read stats is new
data tracked in pg_stat_io, all three are aggregated if
track_wal_io_timing is enabled. The read part matters during recovery
or if a XLogReader is used.

Extra note: more control over if the types of timings calculated in
pg_stat_io could be done with a GUC that lists pairs of (IOObject,IOOp).

Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Author: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Co-authored-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/3opf2wh2oljco6ldyqf7ukabw3jijnnhno6fjb4mlu6civ5h24@fcwmhsgmlmzu

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6c349d83b607cfe20a685a5f66a4fa0e24dbe458

Modified Files
--------------
doc/src/sgml/config.sgml | 32 +++++++++++++++++++++++++--
doc/src/sgml/monitoring.sgml | 31 +++++++++++++++++++++-----
doc/src/sgml/wal.sgml | 4 ++--
src/backend/access/transam/xlog.c | 9 ++++----
src/backend/access/transam/xlogreader.c | 2 +-
src/backend/access/transam/xlogrecovery.c | 2 +-
src/backend/storage/buffer/bufmgr.c | 10 ++++-----
src/backend/storage/buffer/localbuf.c | 4 ++--
src/backend/storage/smgr/md.c | 4 ++--
src/backend/utils/activity/pgstat_backend.c | 3 ++-
src/backend/utils/activity/pgstat_io.c | 14 +++++++-----
src/backend/utils/misc/guc_tables.c | 9 ++++++++
src/backend/utils/misc/postgresql.conf.sample | 1 +
src/include/access/xlog.h | 1 +
src/include/pgstat.h | 2 +-
15 files changed, 96 insertions(+), 32 deletions(-)