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?

[roock]

I also needed to adopt the query for our setup to include more details as we have similar proxy names within multiple environments so it was not possible to find out which environment/host was affected.