Publish checkpoint timing information to pg_stat_bgwriter.

doc/src/sgml/monitoring.sgml

src/backend/access/transam/xlog.c

src/backend/catalog/system_views.sql

src/backend/postmaster/pgstat.c

src/backend/storage/smgr/md.c

src/backend/utils/adt/pgstatfuncs.c

src/include/catalog/catversion.h

src/include/catalog/pg_proc.h

src/include/pgstat.h

src/test/regress/expected/rules.out