Expose config metrics

[kenneth-duffel]

Config metrics are not currently not exposed
by the exporter. These metrics are used in some
existing dashboards.

Config metrics being exposed:

• max_client_conn: pgbouncer_config_max_client_conn
• max_user_connections: pgbouncer_config_max_user_connections

Signed-off-by: kenneth-duffel 93927344+kenneth-duffel@users.noreply.github.com

[kenneth-duffel]

@stanhu and @SuperQ 👋
Can I get a review on this addition please 🙏 .
My team recently started using this exporter and we thought it would be good to add this so some of our dashboards are complete.

[kenneth-duffel]

@SuperQ and @stanhu I have updated the name as per your recommendations. Please review 🙏

[kenneth-duffel]

Thanks you

[alex1989hu]

Dear @SuperQ and @kenneth-duffel I have been using pgbouncer_exporter for ages. I have just rebuilt it from master branch and I see in the logs:

{"caller":"collector.go:474","err":"error retrieving columns list from SHOW CONFIG:  <nil>\n","level":"error","msg":"error getting SHOW CONFIG","ts":"2023-01-26T18:51:11.176Z"}

I have PgBouncer 1.17.0 where I have more than 3 colums:

pgbouncer=# SHOW CONFIG;
            key            |                         value                         |                        default                         | changeable
---------------------------+-------------------------------------------------------+--------------------------------------------------------+------------

It should be at least 3, or do I miss something?

pgbouncer_exporter/collector.go

Lines 189 to 192 in 07052df

	columnNames, err := rows.Columns()
	if err != nil || len(columnNames) != 3 {
	return errors.New(fmt.Sprintln("error retrieving columns list from SHOW CONFIG: ", err))
	}

UPDATE: I found it, so the code expects these 3 columns in this order but I have default column as well before the changeabl value

pgbouncer_exporter/collector.go

Lines 199 to 201 in 07052df

	var key string
	var values sql.RawBytes
	var changeable string

Based on the documentation (https://www.pgbouncer.org/usage.html) there are 4 columns:

SHOW CONFIG
Show the current configuration settings, one per row, with the following columns:

key
  Configuration variable name
value
  Configuration value
default
  Configuration default value
changeable
  Either yes or no, shows if the variable can be changed while running. If no, the variable can be changed only at boot time. 
Use SET to change a variable at run time.