Sample rate added to pg_stat_statements

Hi hackers,

Under high-load scenarios with a significant number of transactions per
second, pg_stat_statements introduces substantial overhead due to the
collection and storage of statistics. Currently, we are sometimes forced
to disable pg_stat_statements or adjust the size of the statistics using
pg_stat_statements.max, which is not always optimal. One potential
solution to this issue could be query sampling in pg_stat_statements.

A similar approach has been implemented in extensions like auto_explain
and pg_store_plans, and it has proven very useful in high-load systems.
However, this approach has its trade-offs, as it sacrifices statistical
accuracy for improved performance. This patch introduces a new
configuration parameter, pg_stat_statements.sample_rate for the
pg_stat_statements extension. The patch provides the ability to control
the sampling of query statistics in pg_stat_statements.

This patch serves as a proof of concept (POC), and I would like to hear
your thoughts on whether such an approach is viable and applicable.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.