Add support for prometheus target relation (for haproxy)

[Alvaro Uria]

HAProxy Community Edition has prometheus metrics exporter support since v2.0 [1] (focal) and the deb package is already built with such capability [2] (unfortunately, bionic provides haproxy v1.8, which is not supported built-in).

charm-hacluster should support a Juju interface so Prometheus can collect metrics from the exported path (all hacluster units need to be scraped, so the real IPs need to be used).

charm-prometheus2 already supports the target relation, so charm-hacluster should react to such relation being created.

Additionally, a relation between cs:grafana and charm-hacluster should exist so Grafana can import a dashboard [3] to paint HAProxy metrics (similarly to charm-prometheus2, cs:grafana also supports an interface for such imports).

References:
1. https://www.haproxy.com/blog/haproxy-exposes-a-prometheus-metrics-endpoint/
2. http://ftp.acc.umu.se/ubuntu/pool/main/h/haproxy/haproxy_2.0.13-2ubuntu0.1.debian.tar.xz
3. https://grafana.com/grafana/dashboards/367