HAproxy: add instance label to sums

[matejzero]

At the moment summaries do not report instance for which the alert is triggered, because we sum only by proxy label and all the rest are stripped.

Example:

      - alert: HaproxyFrontendSecurityBlockedRequests
        expr: sum by (proxy) (rate(haproxy_frontend_denied_connections_total[2m])) > 10
        for: 2m
        labels:
          severity: warning
        annotations:
          summary: HAProxy frontend security blocked requests (instance {{ $labels.instance }})
          description: "HAProxy is blocking requests for security reason\n  VALUE = {{ $value }}\n  LABELS = {{ $labels }}"

This PR adds instance label to all sums, so we get the instance label too.

[samber]

IMO, some queries should be aggregated over your haproxy instances.

For example, if I have a setup of 2 LB, proxying request to an API, I would track the rate of 4xx errors globally. Not at the instance level.

Can you explain this contrib?