replace 2 string.match and 1 string.gmatch (space_split)
by 1 string.match

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

As prometheus-node-exporter-lua is a reimplementation of node_exporter,
I'm using "collector" instead of "scraper" and renaming some collectors

put each collector in a separate file

report collector success/failure and duration per scrape
(follow https://github.com/prometheus/node_exporter/pull/516)

allow to filter collectors using "collect[]" params
(see https://github.com/prometheus/node_exporter#filtering-enabled-collectors)

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>