sl-exporter should be open sourced (and archive Half-life)

[DavidNix]

Once we feel like sl-exporter is good for an MVP release. I suggest open-sourcing it and archive half-life.

It's rare we get feature or support requests for half-life. However, one less project to maintain is a win for SL. Additionally, half-life is not a good solution at scale, and I would not want to encourage teams to use it if they are monitoring more than a handful of nodes.

[jonathanpberger]

Put under apache in 9717694. Closing ticket.