Extend configurability of model count metrics

[Kircheneer]

Environment

• Nautobot version: 1.5.4
• nautobot-capacity-metrics version: 2.0.0

Proposed Functionality

Make it possible to "subscribe" to additional labels for model counts, such as the region name for devices, by config.

Use Case

Allow for more flexibility.

Possible implementation

Extend configuration to include a list of labels to be added like

["site.region.name", "name"]

to include as labels on the relevant model count metric.

[Kircheneer]

We may be able to auto-derive this by model introspection, traversing the relationships and describing the related objects by their natural key (Nautobot 2.0, see here)

[Kircheneer]

Solved by #47. The introspection aspect is quite complicated, especially once you get into traversing multiple relationships, so I would like to avoid it at least for now,