Allows for additional labels to be applied to services

This allows us to set additional labels on services, which GitLab.com needs for changing the job name in metrics for prometheus-operator. gitlab-com/gl-infra/delivery#1167 (closed). When adding custom service monitors in prometheus-operator, the only way to change the job label for a metric is to add a label to the service https://github.com/helm/charts/blob/master/stable/prometheus-operator/values.yaml#L1985-L1988

This follows the same pattern for podLabels in https://gitlab.com/gitlab-org/charts/gitlab/-/blob/8681ede891d68e85c35bf190809740d1afccfd25/templates/_helpers.tpl#L304