Export metrics for php-fpm

Summary:
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

Test Plan: Deployed on WindRiver

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1999

Differential Revision: https://devcentral.nasqueron.org/D3502