Add metric for compute quota `server_group_members`

[MPV]

What

Add a metric for server_group_members quota.

Why

In order not to exceed the quota of "number of Members in each Server Group", we'd like to be able to alert on this.

Notes

References:

• https://docs.openstack.org/nova/pike/admin/quotas.html
• https://github.com/gophercloud/gophercloud/blob/8f11ccebc24d40d6c8032ede7ed724cd09f24553/openstack/compute/v2/quotasets/results.go#L105C40-L105C60

[CLA-Mercedes-Benz]

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