Add Prometheus metrics for the plugin

[gertzakis]

Environment

• Nautobot version: ^1.5.13
• nautobot-circuit-maintenance version: 1.0.0

Proposed Functionality

Expose different Prometheus metrics for circuit maintenance based on Nautobot's implementation for better monitoring.

Use Case

Some metrics as example are:

• Number of circuit maintenances that are on-going
• Number of circuit maintenance that needs acknowledgement
• Circuit maintenance per provider
• Circuit maintenance per circuit