some description please...

The template provides the stats in a machine readable format if you wanted to track connections with a monitoring system like prometheus, telegraf, or fluentd. From there, you could graph connections with grafana or the like.

the file doesn't look like html. shouldnt it have a different extension? link to prometheus for interested parties pls.

https://prometheus.io/docs/instrumenting/exporters/

The official node exporter for linux hosts uses text/plain, so it should be .txt or have no extension.

https://github.com/prometheus/node_exporter example:

> GET /metrics HTTP/1.1
> Host: localhost:9100
> User-Agent: curl/7.71.1
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Content-Type: text/plain; version=0.0.4; charset=utf-8
< Date: Sat, 12 Dec 2020 15:57:58 GMT
< Transfer-Encoding: chunked