pgsql: Fix issues in pg_stat_wal.

Fix issues in pg_stat_wal.

1) Previously there were both pgstat_send_wal() and pgstat_report_wal()
in order to send WAL activity to the stats collector. With the former being
used by wal writer, the latter by most other processes. They were a bit
redundant and so this commit merges them into pgstat_send_wal() to
simplify the code.

2) Previously WAL global statistics counters were calculated and then
compared with zero-filled buffer in order to determine whether any WAL
activity has happened since the last submission. These calculation and
comparison were not cheap. This was regularly exercised even in read-only
workloads. This commit fixes the issue by making some WAL activity
counters directly be checked to determine if there's WAL activity stats
to send.

3) Previously pgstat_report_stat() did not check if there's WAL activity
stats to send as part of the "Don't expend a clock check if nothing to do"
check at the top. It's probably rare to have pending WAL stats without
also passing one of the other conditions, but for safely this commit
changes pgstat_report_stats() so that it checks also some WAL activity
counters at the top.

This commit also adds the comments about the design of WAL stats.

Reported-by: Andres Freund
Author: Masahiro Ikeda
Reviewed-by: Kyotaro Horiguchi, Atsushi Torikoshi, Andres Freund, Fujii Masao
Discussion: https://postgr.es/m/20210324232224.vrfiij2rxxwqqjjb@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d8735b8b4651f5ed50afc472e236a8e6120f07f2

Modified Files
--------------
src/backend/postmaster/checkpointer.c | 4 +-
src/backend/postmaster/pgstat.c | 121 +++++++++++++++++++---------------
src/include/executor/instrument.h | 12 ++++
src/include/pgstat.h | 3 +-
4 files changed, 82 insertions(+), 58 deletions(-)