HAProxy monitoring: Check connection to HAProxy services when adding them for monitoring

Here are two "how to test". One for CLI, another for UI.

How to test CLI:
1. pmm-admin add haproxy haproxy_name --listen-port=555 (or another fault port)
Expected: Connection check failed: Get "http://127.0.0.1:465/metrics": dial tcp 127.0.0.1:465: connect: connection refused.

2. pmm-admin add haproxy haproxy_name --listen-port=REALPORT (real port where HAProxy running, for me it is 8404)
Expected: HAProxy Service added.
Service ID : /service_id/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Service name: haproxy_name

Check screenshot for both cases and theirs results here:

 
How to test UI:
1. Go to grafana
2. Add instance
3. Select HAProxy option
4. Fill some name
5. Try some random port which is not used, it should show error (connection refused etc..)
6. Try port where haproxy running, it should add haproxy with success.
7. Check inventory for haproxy service and external exporter.