repro memory leak under pg_stat_monitor

[steve-chavez]

Problem

When adding pg_stat_monitor like:

shared_preload_libraries='pg_net, pg_stat_monitor'

Memory usage keeps increasing for the pg_net background worker.

To prove this execute in one terminal:

xpg -v 15 psql

And this in another:

watch -n 1 'ps -p $(cat build-15/bgworker.pid) -o %mem,rss,vsz,comm'

%MEM   RSS    VSZ COMMAND
 0.1 19164 722508 postgres

The RSS should be seen as always growing, which proves the memory leak.

Root cause

pg_stat_monitor statistics are split into configurable time intervals (buckets). As one bucket fills or a time interval passes, pg_stat_monitor creates a new one, keeping historical data in distinct buckets.

pg_net queries its internal tables each second.

This means that the "memory leak" is actually expected behavior by pg_stat_monitor.

We need to fix this in pg_net itself, by doing #164.