This commit adds the ironic-prometheus-exporter, following the
conventions used by the previously integrated exporters. '[The] Ironic
Prometheus Exporter is a Tool to expose hardware sensor data in the
Prometheus format through an HTTP endpoint.'[0]

Prometheus has been enabled in CI jobs to ensure test coverage.

[0] https://opendev.org/openstack/ironic-prometheus-exporter

Depends-On: https://review.opendev.org/c/openstack/kolla/+/874415

Change-Id: I6d421effd833d2e0524dd0b81736445c9a730ea9