Review other (non-netobserv) metrics available out there, and see if we can leverage our alerting+health mechanism on them too

E.g:

• ingress errors (haproxy_server_http_responses_total)
• ingress performance degrading (? haproxy_server_http_average_response_latency_milliseconds)
• ingress connections coming close to capacity

To start with, we should probably use RecordingRules rather than Alerts here, to make sure we don't produce unwanted noise.

See also these RFEs: RFE-2935, RFE-8004