Expose metrics in the default format at php-fpm level, then expose them
to Prometheus using an exporter compatible with sockets.
Since PHP 8.1, php-fpm also support OpenMetrics format, but that would
require to switch to a TCP configuration.
This tool needs access to php-fpm sockets, all belonging to web group.
The web-admin user is created to allow that access.
Ref T1999