HAProxy monitoring: Ability to add HAProxy services with `pmm-admin [inventory] add [service] haproxy` command

We should implement Management API and Inventory API for HAProxy.

We should have the same level of support we have for ProxySQL. The only difference will be that we will use built-in support for prometheus metrics in HAProxy v2.

Under the hood, pmm-admin add haproxy will add HAProxy Service (new Service type) and external exporter for that service. Connection checking is a must there.

For testing we can use our repository:

# percona-release enable pdpxc-8.0
* Enabling the Percona Distribution for MySQL 8.0 - PXC repository
<*> All done!

# yum install -q -y percona-haproxy

# haproxy -vv | grep Prometheus
Built with the Prometheus exporter as a service

Management API and Inventory APIs should have all endpoints and fields that External exporters does and should have the same logic as external exporters. With only one difference: agent type should be haproxy.

Suggested implementation:

• CLI
  • `pmm-admin add haproxy --address=<address> --listen-port=<port> --username=<username> --password=<password>`
  • `pmm-admin remove haproxy <service name>`
  • `pmm-admin inventory add service haproxy`
  • `pmm-admin inventory remove service haproxy <service name>`
  • Username and password usage is the same as for external exporters
  • Add flag for skip connection check

How to test:
Case 1 (pmm-admin add haproxy):

• Setup HAProxy with built-in prometheus exporter
• pmm-admin add haproxy
• get list of services via CLI `pmm-admin list`
  • check if we have HAProxy service there and external exporter there
• Check that we are getting metrics from HAProxy via Advanced Data Exploration dashboard in Grafana.
• remove HAProxy via CLI

Case 2 (pmm-admin inventory add service haproxy):

• Setup HAProxy with built-in prometheus exporter
• pmm-admin inventory add service haproxy
• get list of services via CLI `pmm-admin list`
  • check if we have HAProxy service there
• remove HAProxy via CLI

DOC:

• pmm-admin CLI reference manual
• Description about adding HAProxy in documentation