Hello! I have small change suggestion. 
I think its too small to add documentation/changelog note, but if I
should then just say and I can add. :)

**What this PR does / why we need it**: it adds possibility to set
labels for Loki gateway Ingress.

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [ ] Tests updated
- [x] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`