CFP: Add subnet_id to cilium_operator_ipam_ips metric

Cilium Feature Proposal

We're using Cilium with the AWS ENI integration have both a one to many, and many to one mapping between clusters and subnets (i.e. clusters spawn nodes in multiple subnets, and the same subnets is used by multiple clusters). In order to track subnet utilization (especially with prefix-delegation), we'd like to add a label to the cilium_operator_ipam_ips metric with subnet_id, in order to track subnet utilization.

Is your proposed feature related to a problem?

Lack of subnet visibility when using prefix-delegation across multiple clusters in AWS.

Describe the feature you'd like

Add subnet_id to the cilium_operator_ipam_ips metric.

Notify relevant community channels

Notify the members of any relevant code owners below from the teams list in the following form:

• @cilium/example

If you don't know which team to include or you haven't gotten feedback, consider raising this topic on Slack in the #development channel or during a community meeting for awareness.

(Optional) Describe your proposed solution

This seems pretty straightforward. We already have the subnet_id on the cilium_operator_ipam_available_ips_per_subnet metric.

Once the CFP is close to being finalized, please add it as a PR to the design-cfps repo for final approval.