Add metrics alerts for COS integration

### Enhancement Proposal

Currently, only Loki alerts are present in the HAProxy charm code. I propose to enhance the alerting for this charm via COS integration by adding additional rules. I see some good rule candidates in https://samber.github.io/awesome-prometheus-alerts/rules/proxies-load-balancers-and-service-meshes/haproxy/

### Impact

Low (The feature is nice to have)

### Impact Rationale

We can add these to IS COS ourselves via `cos-configuration-k8s` charm, but would be best to live in the charm itself.