gateway port name incorrectly renamed `http-metrics`

[slim-bean]

          For the loki helm chart: https://github.com/grafana/loki/pull/12067 changed the port name for the gateway service from `http` to `http-metrics` which caused it to be picked up by the [loki `ServiceMonitor`](https://github.com/grafana/loki/blob/main/production/helm/loki/templates/monitoring/servicemonitor.yaml).

The gateway responds with a 404 on the /metrics path causing the prometheus target to fail.

Originally posted by @AllexVeldman in https://github.com/grafana/loki/issues/12506#issuecomment-2046899332