Move Exporter to /collector so it can be used as a library

[arvenil]

This decouples collector implementation from http server and cli client.
This way I can create my own binary with several collectors by using collector.New.

All scrappers were already in /collector subpackage, so I just moved Exporter struct which is implementation of prometheus Collector, also to /collector subpackage.

[SuperQ]

Great idea!

[arvenil]

Builds now, I can rebase if you wish.

[brian-brazil]

This way I can create my own binary with several collectors by using collector.New.

This is not a recommended way to deploy things, see https://prometheus.io/docs/instrumenting/writing_exporters/#deployment

[arvenil]

@brian-brazil I know that. It doesn't change anything for prometheus exporters.

It lets me to implement my own http server, with my own modifications... which I will likely try to push upstream if they make sense for you guys, but there might be some that doesn't.

This keeps cli and http interface in one package, and collector in another one - making easier to develop things on top of the collector.

Exporter is implementation of prometheus Collector interface which I can't access because it's in main package. It should also make unit testing easier.

[SuperQ]

Looks like this needs a rebase.

[arvenil]

Done.

[SuperQ]

LGTM

[SuperQ]

@grobie PTAL

[grobie]

LGTM

[frimik]

nevermind, my bad!