Create simple endpoint for health checking

Health checking / or /metrics doesn’t like a very good idea, especially when dealing with large cluster that contain many keyspaces and data.

[deimosfr]

/health should just say the exporter is working properly (print “ok” if everything is fine)

[erebe]

Won’t do it as :

1. Adding a new endpoint will be complicated, as for now the http server is embeded in the prometheus library and I don’t have access to it at the level of the app
2. I feel the /health endpoint is redundant (but I may be biased due to the point 1) as you can do HEAD request or use the filtering feature to get a simple /health without any impacts on the app

As always, PR are welcome 😃