Fixed lint warnings in Grafana raised by Mixtool (#22486)

Fixed lint warnings in Grafana raised by Mixtool (#22486)

This PR introduces a few minor changes to the gitea-monitoring-mixin,
specifically linting issues raised by
[Mixtool](https://github.com/monitoring-mixins/mixtool):
- Query selectors using `job` and `instance` have been update to allow
multi-select
- Added missing attributes to `job` and `instance` template

As this change is very minor I haven't created an issue, but please let
me know if you'd like me to do so. According to the guidelines, it
seemed to only be for larger designs :)