pgsql: Adjust tuplestore stats API

Adjust tuplestore stats API

1eff8279d added an API to tuplestore.c to allow callers to obtain
storage telemetry data.  That API wasn't quite good enough for callers
that perform tuplestore_clear() as the telemetry functions only
accounted for the current state of the tuplestore, not the maximums
before tuplestore_clear() was called.

There's a pending patch that would like to add tuplestore telemetry
output to EXPLAIN ANALYZE for WindowAgg.  That node type uses
tuplestore_clear() before moving to the next window partition and we
want to show the maximum space used, not the space used for the final
partition.

Reviewed-by: Tatsuo Ishii, Ashutosh Bapat
Discussion: https://postgres/m/CAApHDvoY8cibGcicLV0fNh=9JVx9PANcWvhkdjBnDCc9Quqytg@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/9fba1ed2947382af213dfbfabfcd8898c89bf4ca

Modified Files
--------------
src/backend/commands/explain.c      | 17 +++++-----
src/backend/utils/sort/tuplestore.c | 64 +++++++++++++++++++++----------------
src/include/utils/tuplestore.h      |  5 ++-
3 files changed, 48 insertions(+), 38 deletions(-)