Add metric total_autovacuum_time

[konrad-erli]

Proposal

Export new columns of pg_stat_user_tables, that were introduced in postgres 18:
total_vacuum_time, total_autovacuum_time, total_analyze_time, total_autoanalyze_time

Rate of total_autovacuum_time (over longer periods) shows how much vacuuming does a table require, shows if table is putting pressure on autovacuum system. Analyzing it helps tune autovacuum settings: number of threads, budgests/sleeps.