Add metrics for ClusterQueue usage

[alculquicondor]

What would you like to be added:

Usage metrics for ClusterQueue quotas.

To uniquely identify the resources, we need the following labels: cluster_queue, flavor, resource.

Note that, depending on the number of ClusterQueues, ResourceFlavors and resources, these set of labels could have a big cardinality. For this reason, we need to make this metric opt-in. Additionally, we need to make sure to delete the metrics as ClusterQueues get deleted.

Why is this needed:

Administrators need these metrics to build dashboards and track usage.

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[trasc]

/assign