feat: expose usage counters for prepared statements

[sboschman]

With prepared statements tracking enabled in PgBouncer we internally track the following counters:

• ps_client_parse_count
• ps_server_parse_count
• ps_bind_count

But we only expose them in log/systemd messages, but not in any SHOW command, which are used by external monitoring tools like pgbouncer_exporter. These counters are imo important to gain insight in how well 'prepared statement caching' in PgBouncer is working for your workload and your configuration. I think of them as a sort of 'cache hit ratio'.

[sboschman]

@JelteF hopefully you have time to take a look at this addition to the prepared statement implementation

[JelteF]

LGTM, but needs some manual testing on my or someone else's side.

[JelteF]

If you could add some tests for this that would be great btw.

[sboschman]

If you could add some tests for this that would be great btw.

What kind of tests are you thinking about? The existing test_admin tests already check the SHOW commands. So f.e. a mismatch between header columns and data columns is detected.

[JelteF]

Basically a modification to test_show_stats() to show that the returned counts make sense:

pgbouncer/test/test_admin.py

Line 226 in 585a630

def test_show_stats(bouncer):

[sboschman]

A small test case has been added to the prepared statement test suite: test_prepared.py

Anything else I can do to help out on this issue @JelteF ?