Add http support

[Qwiko]

Renamed my branch and accidentally deleted my previous pull request. Proposed solution for: #33 and #32 (Not all metrics added yet)

Tested towards kea version 2.4

Changelog

• Moved shared resources to BaseExporter class
• Created a unified parse_metrics function that both modes use
• Updated socket-exporter to use BaseExporter-class and parse_metrics
• Implemented a HTTP exporter
• Uses logic to handle multiple services when pulling http statistics, instead of pulling one service at a time.
• Has the same issue where it is not known if the configuration have been changed so we check every time we reload the metrics.
• Updated CLI to support multiple modes, default mode == socket
• Added pool metrics Example pool-metrics: If pool == "" it is global metrics for the entire subnet.

  kea_dhcp4_addresses_assigned_total{pool="",subnet="10.10.111.0/24",subnet_id="476"} 7.0
  kea_dhcp4_addresses_assigned_total{pool="10.10.111.200-10.10.111.254",subnet="10.10.111.0/24",subnet_id="476"} 7.0

[skorpy2009]

Works for me :+1: