fix: Aggregation labels for gitaly_active_node_available_space

This makes sure we use the correct aggregation labels for thegitaly_active_node_available_space component. Previously, this did not include the required stage='main'label. Meaning we weren't reporting on these metrics in tamland or capacity planning since that is part of the selector.

This is where the selector is used for dashboards: https://gitlab.com/gitlab-com/runbooks/blob/5c4d420aae2964daf26418664152cc0c0493dd99/dashboards/alerts/saturation_alerts.libsonnet#L10 Here it gets added in tamland: https://gitlab.com/gitlab-com/gl-infra/tamland/blob/e9f10a4a73cf3e70ce04700df1d91a9d592879e2/metadata.py#L66

For gitlab-com/gl-infra/scalability#1727 (closed)