pgsql: Preserve n_live_tup when VACUUM cannot estimate it

Preserve n_live_tup when VACUUM cannot estimate it

VACUUM clamped an unknown live-tuple estimate (-1) to zero before
reporting it to pgstat, overwriting any existing n_live_tup estimate
with zero. It did so because some users required non-negative values and
the situation was seen as rare.

Commit b46e1e54d made the sitaution more likely by allowing on-access
pruning to mark pages all-visible before VACUUM or ANALYZE establishes
tuple density.

Pass -1 through to pgstat_report_vacuum(), which will preserve the
existing n_live_tup when the estimate is unknown.

Reported-by: Vaibhav Jain <jainva(at)google(dot)com>
Author: Vaibhav Jain <jainva(at)google(dot)com>
Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://postgr.es/m/flat/CABa-Az2YXuX%2B_JH8VL89i7oA%2BDC74GvfkSsM9%2B92fGCeWYVCqg%40mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9693c831c1f5fbc2ff437e42a93b0b6274b82105

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 2 +-
src/backend/utils/activity/pgstat_relation.c | 6 +++++-
2 files changed, 6 insertions(+), 2 deletions(-)