[Feature]: Add ability to monitor custom values

Hi,

Long time Cacti user here, but we all know that Cacti can sometimes get to be hairy.

I tried beszel and the experience is great.

From some research I could not find a way to, for example count http connections to my servers. So...

My suggestion is having a VERY simple plugin system where the agent would read a directory of files before sending them to the server and these files are named for example: http_num_conns_min, sql_exec_min, etc.

These files are overwritten by scripts that are written in whatever language the system admin likes and are also reusable from other systems such as Cacti.

On the beszel server side we would see the graph of http_num_conns_min and maybe have an option to put an alias of "HTTP connections per minute" and other metadata if you feel like adding some more inputs.

Let me know if you like this and/or close this enhancement issue if it's a duplicate.

Bless you for writing software like this and keep up the great work!