### Summary
When configuring the Blackbox Exporter/FRR Exporter/Node Exporter service to use an IPv6 address as the `listen-address`, the service fails to start and reports an error due to improper formatting of the address. The error message indicates that there are "too many colons in address," which is a result of the systemd unit not enclosing the IPv6 address in brackets.
This could be considered to be a regression of https://vyos.dev/T4680, and caused by the monitoring services now uses a new template rather than .
### Error Message
The following error is logged when the Node Exporter attempts to start with an IPv6 address:
```
Sep 07 17:15:55 <redacted> node_exporter[6798]: time=2025-09-07T17:15:55.599Z level=ERROR source=node_exporter.go:247 msg="listen tcp: address XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:1:9100: too many colons in address"
```
### Systemd Unit Status
The service status indicates a failure:
```
# systemctl status node_exporter
× node_exporter.service - Node Exporter
Loaded: loaded (/etc/systemd/system/node_exporter.service; disabled; preset: enabled)
Active: failed (Result: exit-code) since Sun 2025-09-07 17:15:55 UTC; 31s ago
Duration: 386ms
Docs: https://github.com/prometheus/node_exporter
Process: 6798 ExecStart=/usr/sbin/node_exporter --web.listen-address=XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:1:9100 (code=exited, status=1/FAILURE)
Main PID: 6798 (code=exited, status=1/FAILURE)
CPU: 340ms
```
### Root Cause
The issue arises from the systemd service unit template `service.j2` located at [this path in vyos-1x](https://github.com/vyos/vyos-1x/tree/current/data/templates/prometheus). The template does not include checks for IPv6 addresses and fails to add the necessary brackets (`[]`) around the address.
### Steps to Reproduce
To replicate the issue, use one of the following configurations:
```
set service monitoring prometheus blackbox-exporter listen-address ::1
set service monitoring prometheus frr-exporter listen-address ::1
set service monitoring prometheus node-exporter listen-address ::1
```
### Suggested Fix
Modify the `service.j2` template to include ` | bracketize_ipv6` for the `{{ address }}` (so it becomes `{{ address | bracketize_ipv6 }}`) so that it automatically adds brackets around IPv6 listen-addresses. This will ensure that the various exporter service can start successfully when configured with an IPv6 address.