pgsql: Fix briefly showing old progress stats for ANALYZE on inherited

Fix briefly showing old progress stats for ANALYZE on inherited tables.

ANALYZE on a table with inheritance children analyzes all the child
tables in a loop. When stepping to next child table, it updated the
child rel ID value in the command progress stats, but did not reset
the 'sample_blks_total' and 'sample_blks_scanned' counters.
acquire_sample_rows() updates 'sample_blks_total' as soon as the scan
starts and 'sample_blks_scanned' after processing the first block, but
until then, pg_stat_progress_analyze would display a bogus combination
of the new child table relid with old counter values from the
previously processed child table. Fix by resetting 'sample_blks_total'
and 'sample_blks_scanned' to zero at the same time that
'current_child_table_relid' is updated.

Backpatch to v13, where pg_stat_progress_analyze view was introduced.

Reported-by: Justin Pryzby
Discussion: https://www.postgresql.org/message-id/20230122162345.GP13860%40telsasoft.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5ae245664830219bab2e2ada11d2379875f4501a

Modified Files
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src/backend/commands/analyze.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)