### Purpose
Adds a new metric `syncthing_connections_active` which equals to the
amount of active connections per device.
Fixes#9527
<!--
Describe the purpose of this change. If there is an existing issue that
is
resolved by this pull request, ensure that the commit subject is on the
form
`Some short description (fixes#1234)` where 1234 is the issue number.
-->
### Testing
I've manually tested it by running syncthing with these changes locally
and examining the returned metrics from `/metrics`.
I've done the following things:
- Connect & disconnect a device
- Increase & decrease the number of connections and verify that the
value of the metric matches with the amount displayed in the GUI.
### Documentation
https://github.com/syncthing/docs/blob/main/includes/metrics-list.rst
needs to be regenerated with
[find-metrics.go](https://github.com/syncthing/docs/blob/main/_script/find-metrics/find-metrics.go)
## Authorship
Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.
---------
Co-authored-by: Jakob Borg <jakob@kastelo.net>
