Praefect: support of PgBouncer

As Praefect talks to PostgeSQL instance we should use PgBouncer
in front of database to reduce resource consumption.
All required changes already done https://gitlab.com/gitlab-org/gitaly/-/issues/2816
in Praefect and tested https://gitlab.com/gitlab-org/gitaly/-/issues/2975
on the weekly demo.
The change describes required options that should be set for the
PgBouncer instance and what values should be substituted so
Praefect instances could connect to PostgreSQL throw PgBouncer.

Closes: https://gitlab.com/gitlab-org/gitaly/-/issues/2976

doc/administration/gitaly/praefect.md

@@ -188,6 +188,49 @@ node, using `psql` which is installed by Omnibus GitLab.
 
 The database used by Praefect is now configured.
 
+#### PgBouncer
+
+To reduce PostgreSQL resource consumption, you should set up and configure
+[PgBouncer](https://www.pgbouncer.org/) in front of the PostgreSQL instance. To do
+this, replace value of the `POSTGRESQL_SERVER_ADDRESS` with corresponding IP or host
+address of the PgBouncer instance.
+
+This documentation doesn't provide PgBouncer installation instructions,
+you can:
+
+- Find instructions on the [official website](https://www.pgbouncer.org/install.html).
+- Use a [Docker image](https://hub.docker.com/r/edoburu/pgbouncer/).
+
+In addition to base PgBouncer configuration options, set the following values:
+
+- The [Praefect PostgreSQL database](#postgresql) in the `[databases]` section:
+
+   ```ini
+   [databases]
+   * = host=POSTGRESQL_SERVER_ADDRESS port=5432 auth_user=praefect
+   ```
+
+- [`pool_mode`](https://www.pgbouncer.org/config.html#pool_mode)
+  and [`ignore_startup_parameters`](https://www.pgbouncer.org/config.html#ignore_startup_parameters)
+  in the `[pgbouncer]` section:
+
+   ```ini
+   [pgbouncer]
+   pool_mode = transaction
+   ignore_startup_parameters = extra_float_digits
+   ```
+
+The `praefect` user and its password should be included in the file (default is
+`userlist.txt`) used by PgBouncer if the [`auth_file`](https://www.pgbouncer.org/config.html#auth_file)
+configuration option is set.
+
+NOTE: **Note:**
+By default PgBouncer uses port `6432` to accept incoming
+connections. You can change it by setting the [`listen_port`](https://www.pgbouncer.org/config.html#listen_port)
+configuration option. We recommend setting it to the default port value (`5432`) used by
+PostgreSQL instances. Otherwise you should change the configuration parameter
+`praefect['database_port']` for each Praefect instance to the correct value.
+
 ### Praefect
 
 To complete this section you will need: