MINOR: promex: Add agent check status/code/duration metrics

MINOR: promex: Add agent check status/code/duration metrics

In the Prometheus exporter, the last health check status is already exposed,
with its code and duration in seconds. The server status is also exposed.
But the information about the agent check are not available. It is not
really handy because when a server status is changed because of the agent,
it is not obvious by looking to the Prometheus metrics. Indeed, the server
may reported as DOWN for instance, while the health check status still
reports a success. Being able to get the agent status in that case could be
valuable.

So now, the last agent check status is exposed, with its code and duration
in seconds. Following metrics can be grabbe now:

  * haproxy_server_agent_status
  * haproxy_server_agent_code
  * haproxy_server_agent_duration_seconds

Note that unlike the other metrics, no per-backend aggregated metric is
exposed.

This patch is related to issue #2983.